You should have at least 4 or 5 square feet per bird. This is a general rule, and not carved in stone, but more is better. So, for 6 chickens, start at 30 square feet. Then you must apply chicken math. You have to allow for future growth so double that, say 60 square feet of floor space. Now you must consider the lumber sizes. A standard board or piece of plywood is 8 feet long, no sense in making it any shorter than that, you'd just have to cut some off. If you make it 4' x 8' then it would be too small (32 square feet) so you have to double that. Now you are up to 8' x 8' or 64 square feet. That would be just floor space for birds, there is also space for feeders and waterers and nest boxes, not to mention feed storage and bedding storage and all the other stuff you will need to keep handy. It gets bigger and bigger, and of course the bigger the coop the more chickens you can get and the more feed you need to store..........
Honestly, make it just a little bigger than your space and budget will allow............then add a little more!