I plan on having 4 large hens in my 15x15 townhouse backyard. I am having them in an eglu with a homemade run that is 3x15 with the eglu run attached to that. The Eglu makes the most sense for my yard, but I have also considered making a coop out of one of those big plastic rubbermaid deck boxes. I think my chickens will be just fine back there. My veggie garden is going to be above them in a huge planter box. It seems to me that people think the chickens need a lot more space than they really do, but the small coops I see on here take up no more of a footprint than my set-up will.
I see no point in having a lot of wasted space in my backyard, so I see no problem with taking up that much of it with the chickens. My yard has no grass, the chickens will be on bark chips.
Have fun with your backyard chickens.