How big of a brooder area you need is a function of how many chicks you will have and how old they will be when you are ready to move them out. My brooder is 3' x 6' and in the coop. I regularly raise 20 or more in there until they are five weeks old. The most I think I've had in that brooder was 25. They were never crowded but were out by 5 weeks when I had that many. I'd expect to be able to keep 20 in there until they were over 6 weeks old.
I'll include a photo, I think that makes it easier to talk about. I keep one end warm but let the far end cool off as it will. I put chicks straight from the incubator or post office in here, whether it is below freezing or the heat of summer. My goal is to keep one area warm enough in the coolest conditions and an area cool enough in the warmest conditions. I don't know what your garage was like but out here I've seen it go from below freezing to the 70's Fahrenheit in less than a day and a half. To me that's the biggest challenge to brooding outside, the temperature swings.
Heat plates, heating pads, and such will work fine, but how many chicks will fit, especially when they are 3 or 4 weeks old? I use heat lamps securely wired in place. If you go with heat lamps I strongly suggest you remove the clamp so you are not tempted to use it. Secure the heat lamp with wire or chain so it can't accidentally fall. Not string or plastic that can burn or melt, but wire or chain. To me properly securing it gets rid of the vast majority of risk compared to using that clamp.
If I brood when it can freeze that plastic wrap goes all the way to the top. I want to keep the heat in and especially stop all breezes. In warmer weather I do something closer to this. That "chimney" to the left still gives good ventilation as hot air rises so the far end can cool off. If I were building this again I'd have the ventilation on the far end, away from the heat lamp. You can vary the heat output by changing the wattage of the bulb, in summer I use a 75 watt bulb. In the dead of winter I use a 250 watt bulb and use two heat lamps for redundancy. Even straight from the incubator my chicks have been very good about being able to find comfortable temperatures as long as they have the option. When they are young and it is really cold they spend a lot of their time in the heated area. When they got a bit older they spread out more.
One of your issues when you brood in freezing temperatures is that water freezes. I use a waterer that looks something like this. This is in my grow-out coop, not the brooder. When it is in the brooder I fill it with rocks so they can "walk on water" instead of falling in and getting soaked. That platform is big enough that they can hop up there and stand to drink but it helps keep the water cleaner from bedding or trash. They will poop in it so the water needs to be dumped daily. Since the black bowl is rubber if the water freezes you can break the ice out without breaking the bowl. I keep it in the area of the heat lamp to help keep water thawed.
Instead of using totes when brooding outside I'd be inclined to build a brooder box. I'd be concerned about a tote getting too hot in warmer weather if you use a heat lamp. With heat plates or heating pads it's less of a concern. You would need to be able to reach inside for cleaning and managing it so you don't want it too big but you want it big enough that they can get away from the heat if they need to. I'd probably want to lock them in the brooder itself for a while until they get used to it as home and are able to always get back to the heat. You can keep them in that brooder box until you are ready to let then have the rest of the grow-out area.
My brooder is elevated with a 1/2" hardware cloth floor. I put bins underneath for the poop to fall in. This makes it really easy to clean and keeps the brooder very dry. When I use it in winter I put a piece of plywood on the floor to hold the heat in. I tip that plywood so poop goes through the wire floor. I don't think you need to elevate it, that does cause some issues, but this also doubles as a broody buster when I don't have chicks in it. There are trade-offs with everything.
Since my brooder is in the coop my chicks essentially grow up with the adults so integration is a piece of cake. I just open the brooder door and let them join the flock. But I have the weather and the outside room where they can all be outside and separated every day of the year. You wont have that weather in Ohio. By outside room mine have at least 60 square feet per chicken even when it is crowded. You have that kind of room as long as snow doesn't make it unavailable. Just another complication for you.
Maybe you are planning on this, but I'd have a section of run associated with that grow-out area so the chicks can get outside and be seen by the adults before you try to integrate them. You could section off a part of your 50 x100 run for that or build a new run specially for that grow-out coop. Have a human-sized gate between the two run sections that you can close or lock open. That gives yo a lot more flexibility in managing them. By the time they start to get crowded they can go outside and still be separated from the adults.
A 10' x 10' building may sound big but once you start sectioning it into storage areas and breeding pens it's not. You need enough room to move around in there, walls get in your way. A storage area is a good idea for feed and various brooding supplies, but I'd be tempted to build a small area in a corner with a door that I can access through that door. Save as much space as you can.
Having a breeding pen in there is good, but fix the door so that you can leave it open and make the entire area available when you can. Try to build it as flexible as you can and give yourself access. To me, your comfort and convenience is just as important as anything you do for the chickens.
Hopefully I've given you some tings to think about. Good luck.
I'll include a photo, I think that makes it easier to talk about. I keep one end warm but let the far end cool off as it will. I put chicks straight from the incubator or post office in here, whether it is below freezing or the heat of summer. My goal is to keep one area warm enough in the coolest conditions and an area cool enough in the warmest conditions. I don't know what your garage was like but out here I've seen it go from below freezing to the 70's Fahrenheit in less than a day and a half. To me that's the biggest challenge to brooding outside, the temperature swings.
Heat plates, heating pads, and such will work fine, but how many chicks will fit, especially when they are 3 or 4 weeks old? I use heat lamps securely wired in place. If you go with heat lamps I strongly suggest you remove the clamp so you are not tempted to use it. Secure the heat lamp with wire or chain so it can't accidentally fall. Not string or plastic that can burn or melt, but wire or chain. To me properly securing it gets rid of the vast majority of risk compared to using that clamp.
If I brood when it can freeze that plastic wrap goes all the way to the top. I want to keep the heat in and especially stop all breezes. In warmer weather I do something closer to this. That "chimney" to the left still gives good ventilation as hot air rises so the far end can cool off. If I were building this again I'd have the ventilation on the far end, away from the heat lamp. You can vary the heat output by changing the wattage of the bulb, in summer I use a 75 watt bulb. In the dead of winter I use a 250 watt bulb and use two heat lamps for redundancy. Even straight from the incubator my chicks have been very good about being able to find comfortable temperatures as long as they have the option. When they are young and it is really cold they spend a lot of their time in the heated area. When they got a bit older they spread out more.
One of your issues when you brood in freezing temperatures is that water freezes. I use a waterer that looks something like this. This is in my grow-out coop, not the brooder. When it is in the brooder I fill it with rocks so they can "walk on water" instead of falling in and getting soaked. That platform is big enough that they can hop up there and stand to drink but it helps keep the water cleaner from bedding or trash. They will poop in it so the water needs to be dumped daily. Since the black bowl is rubber if the water freezes you can break the ice out without breaking the bowl. I keep it in the area of the heat lamp to help keep water thawed.
Instead of using totes when brooding outside I'd be inclined to build a brooder box. I'd be concerned about a tote getting too hot in warmer weather if you use a heat lamp. With heat plates or heating pads it's less of a concern. You would need to be able to reach inside for cleaning and managing it so you don't want it too big but you want it big enough that they can get away from the heat if they need to. I'd probably want to lock them in the brooder itself for a while until they get used to it as home and are able to always get back to the heat. You can keep them in that brooder box until you are ready to let then have the rest of the grow-out area.
My brooder is elevated with a 1/2" hardware cloth floor. I put bins underneath for the poop to fall in. This makes it really easy to clean and keeps the brooder very dry. When I use it in winter I put a piece of plywood on the floor to hold the heat in. I tip that plywood so poop goes through the wire floor. I don't think you need to elevate it, that does cause some issues, but this also doubles as a broody buster when I don't have chicks in it. There are trade-offs with everything.
Since my brooder is in the coop my chicks essentially grow up with the adults so integration is a piece of cake. I just open the brooder door and let them join the flock. But I have the weather and the outside room where they can all be outside and separated every day of the year. You wont have that weather in Ohio. By outside room mine have at least 60 square feet per chicken even when it is crowded. You have that kind of room as long as snow doesn't make it unavailable. Just another complication for you.
Maybe you are planning on this, but I'd have a section of run associated with that grow-out area so the chicks can get outside and be seen by the adults before you try to integrate them. You could section off a part of your 50 x100 run for that or build a new run specially for that grow-out coop. Have a human-sized gate between the two run sections that you can close or lock open. That gives yo a lot more flexibility in managing them. By the time they start to get crowded they can go outside and still be separated from the adults.
A 10' x 10' building may sound big but once you start sectioning it into storage areas and breeding pens it's not. You need enough room to move around in there, walls get in your way. A storage area is a good idea for feed and various brooding supplies, but I'd be tempted to build a small area in a corner with a door that I can access through that door. Save as much space as you can.
Having a breeding pen in there is good, but fix the door so that you can leave it open and make the entire area available when you can. Try to build it as flexible as you can and give yourself access. To me, your comfort and convenience is just as important as anything you do for the chickens.
Hopefully I've given you some tings to think about. Good luck.
