There isn't a size limit for the coop, but I want it small enough to be able to clean it, and I found a great deal on resin sheds in that size that I'm going to make into a coop.
My 7x7 coop is a resin shed. Not the best choice I have made. Most of them do not have windows. The ones that do don't open. It makes it very warm in the summer since they are not insulated. Some of the Lifetime brand sheds do state (on HDs website) that they have sliding windows but I assume they do not have screens. You will want to attach hardware cloth on the windows to keep preds out, which brings up another issue: how do you attach the hardware cloth to the plastic shed? Drill holes in it? Since the windows slide open, I think you would have to put the cloth on the outside of the shed so you don't interfere with the window opening.
You cannot hang much from the walls because they only come with a few brackets to hang things, and you need the special brackets unless you want to drill holes in the plastic. The plastic walls are not very strong to hold up much weight like nest boxes but they handle 2 roosts ok. Make sure you have enough roosting space for your girls also. They may "require" only so many inches per birds, but there is a pecking order and some of the dominant birds might knock the lesser ones off their roost. I have two 7' roosts but only 5 or so birds would sleep on the higher roost. The rest would get knocked down. The other girls do not like the lower roost but a few used it. .
The resin sheds are also very cold in the winter because they are not insulated. I had to hang a heat lamp from the rafters. IF you do this, wrap the cord around the rafters a few times so the lamp will not fall on the floor if they knock it down. And put the lamp way up high (but not too close to the roof) out of the way (not directly over a roost) because they will manage to hit it in their efforts to get a good sleeping spot. Chickens are not graceful.
Not everyone on here will agree with me on using a heat lamp in the winter. It can be dangerous without a lot of precautions so weigh your winter heating options carefully. But any heating source can be dangerous in a coop. Chickens can knock heaters over, wood chips can burn, dust will cake on heaters and bulbs and clog ventilation openings. There are a lot of things to consider when winter comes.
As far as cleaning the coop out, putting poop boards beneath the roosts will really cut down on the amount of times you have to clean out the entire coop. Just rake or shovel the poop into a bucket every day or two and you're good. The poop will freeze in the winter which will make this impossible, so keep piling the wood chips on, then in the spring, bring in a wheelbarrow and fill it up. Another great reason to have a big coop with extra wide doors- wheelbarrows are your friend.
If I could do it all over, I would have put my money into a large well insulated wood coop with lots of windows that open (and some skylights) and had either a garage door or a set of double doors. And an exhaust fan with a thermometer. A well insulated coop rarely needs to be heated in the winter or cooled in the summer, but it does need good ventilation year round, especially in the winter. HTH
hehe 
I always struggle to keep up with the under the roost area. This will make things sooo much easier.
