General accepted rule of thumb for coop space is 4sq ft inside space per large fowl bird, 2 sq ft per bantam fowl. For the run is 10sq ft per bird.
Coop size and style depends on the size of your yard/property and where you want to go with your bird hobby... do you want to have 1 or 2 broody hens raise chicks in the spring? Do you want to have one or two pure breeds which you want to concentrate on and maybe sell hatching eggs or chicks for? Are your kids going to be using them for 4H projects? All of these things can effect how much coop you want and what style.
If you want 5 or 6 hens for family eggs and yard candy (a small mixed flock is a beautiful addition to any yard!) then a premade decorative coop of about 4x6 or 4x8 will work fine, and a 10x10 run... if you want to do any of the things I mentioned above then I suggest what I call a 'walk in coop'... meaning a coop that literally allows you to walk into it and rearrange things and work inside of. Think a large garden shed with a few modifications.
Chickens are messy, and you tend to end up with a lot of tools and clutter related to their care... building a shed large enough to have a 'people area' and a 'chicken area' is often the easiest way to keep things organized and in one place. For mine we walk in through a 'man door' to an area which is about 3 or 4 foot deep, at that point there is a chicken wire wall built on 2x2 frames with doors through into the 'chicken area'. The 'people area' allows us to store a lot of our feed, scratch, first aid, flashlights, spare waterers, a few towels and bowls for different things and cleaning tools. The 'people area' also doubles as a safe spot for broodies or birds who need isolated due to injury who don't really have to go to the 'hospital' cage in the house.
So basically, think of what you need for birds... do you need extra yard storage anyway? If so then go big and get it all in one... you can split shed space into sections based on what you need (but I would never store anything with fuel in the same shed as birds due to fume risks). Added benefit of going with a standard shed is that if you decide not to keep birds you still have a good shed and you aren't trying to sell a used coop....
