If you go to the top of this page and hit the ”coops” tab you can find a lot of different coops, many with material lists and plans. Some people are better about detailing their build than others.
Another option is to get a prefab building at Lowe’s, Home Depot, are maybe a building supply place. Wood normally works better for these but you can use metal. They are not tremendously hard to assemble or you can possibly get someone to assemble it for you through that store. Don’t forget the foundation, you’ll either have to do that yourself or get someone to do it.
If you are in the US you can check Craigslist or something like that for materials or maybe even an entire building you can either move or tear down and rebuild. If you have a “habitat” store close you may find inexpensive windows, doors, vents or other parts.
There are a lot of different things that go into how much room you need. There is no one magic number for space that covers everyone. Calgary Canada will have different requirements than Miami Florida, for example. How you manage them and your flock make-up makes a difference. Chickens don’t understand the concept of room in the coop. They understand the concept of room, period. That room can be in the coop alone, coop and run if the run is available when they need it, or they can sleep in trees and have the whole outdoors.
If you wish you can follow the link in my signature to get some of my thoughts on what makes up room requirements. I don’t give you hard and fast numbers, more things to consider. I find the more room I give them the fewer behavioral problems I have to deal with, the more flexibility I have to deal with problems, and I don’t have to work as hard. I like making my life easier. You can get by with less space but you may pay a price.
For that many chickens I recommend a walk-in coop. You need to be able to reach all parts of the inside of the coop for cleaning, maintenance, possible modifications, or to retrieve and egg or maybe an injured or dead chicken. It’s really hard to achieve that with one of those elevated coops you cannot walk into.
If you are building it from scratch the most inexpensive building materials normally come in 4’ or 8’ lengths. You can usually have less cutting and waste if you plan with these dimensions in mind. Your roof needs to be slanted so rainwater runs off plus overhangs are nice so you can put ventilation up high without rain blowing in so that can mess up your standard dimensions. A 6’ dimension isn’t horrible. You’ll mainly be wasting two feet of 2x4’s probably, but maybe you can use those to build nests. If you are using salvaged material with different base sizes, of course consider that.
I don’t know what climate you are dealing with or anything about how you are going to manage them so I’m guessing a bit. I suggest you use a minimum of 6’ x 8’ or if you live in a country where those are not standard sizes for building materials, look for the equivalent. You could probably go 8’ x 8’ with very little extra expense if you are building it yourself and give you some extra flexibility in case you want to add chickens later, integrate replacements, or if you plan to have a broody hen raise chicks with the flock. If you have really rough weather where they go stretches where they can’t get out much, go with the larger size.
Don’t think of the coop in isolation, it’s part of your space system along with the run if you have a run and the run is available when they are awake. It’s not just weather that might make the run not available. If you lock them in the coop at night for extra predator protection, which a lot of us do, you need to be out there fairly early every day (including Saturdays where you might want to sleep in) or the run is not available. If the run is permanently available when they are awake you can get by with less space but remember to give yourself room to work in there. There are days I like to sleep in, especially if I’m sick.