FizzyZow:
According to the majority of folks on these boards you need about 4 sq. ft. of interior space per bird and 10 sq. ft. of exterior run space. The interior space can be reduced if the chickens have a large area to run in and are in the coop only at night. I might add, DON'T DO IT!!! DON't BUILD FOR JUST TWO!! Only kidding, but I thought four would work just right for me and now the wife and grandkids want more and I will have to expand and mine have been in their 4x4 coop for less than a year. Mine have a ton of room to roam during the day. 640 sq. ft run for 4 BOs. So I could add a couple more to the 16 sq. ft. coop. But the whole family is turning chicken crazy so I will probably build a larger coop.
Good luck.