You'll always want more, so make it as big as you can.
My 12 x 12 ft horse stall turned coop could hold 36 chicks if you used the 4 sq ft per bird rule (12 x 12 = 144 % 4 = 36).
But I must say, I wouldn't want to stick that many birds in there. It's really nice to go out and sit and visit...