As another poster advised, the rule of thumb for full sized birds is 4 sq. ft. in the coop and 10 sq. ft. in the run. The Learning Center here at BYC is a great place to get all sorts of info starting out.
One thing I'd like to add that isn't always addressed is storage space. If you figure the size of your coop based on how many birds you ultimately plan on having, please add in extra space to account for feed storage, supplies, a few bags of pine shavings or bales of straw, etc. It's amazing how quickly the space in a coop can be eaten up with supplies.
"Chicken Math" is something you'll want to take into consideration, too. That's when you plan on getting 15 birds but you ultimately end up with 30. Not unusual that this occurs and it can happen almost overnight. Chickens are very enjoyable and are a bit like potato chips - you can't have just one. Or 15. ;-)
Best of luck!
One thing I'd like to add that isn't always addressed is storage space. If you figure the size of your coop based on how many birds you ultimately plan on having, please add in extra space to account for feed storage, supplies, a few bags of pine shavings or bales of straw, etc. It's amazing how quickly the space in a coop can be eaten up with supplies.
"Chicken Math" is something you'll want to take into consideration, too. That's when you plan on getting 15 birds but you ultimately end up with 30. Not unusual that this occurs and it can happen almost overnight. Chickens are very enjoyable and are a bit like potato chips - you can't have just one. Or 15. ;-)
Best of luck!