Frankly, build the biggest you can afford.
We just moved into a new house and it came with an unfinished 11x19' shed with a 10'+ ceiling. I am in the process of finishing the inside with insulation and drywall and plan on installing a cyclorama on the far end. I'm going to paint the whole thing 18% grey. Its barely big enough for full length portraits but I can always stand outside the double doors on the camera end.
