Mine is 16x16 but it doubles as a storage area too.
Just make sure that you have enough air movement for boiling, I have a gable vent fan on one end and keep the door open on the other side, but it still get hotter than hell in the summer (hit 108 when I was brewing the other day).
Have at least true 8' ceiling/rafters so if you do AG 3 -tier you have some room overhead:
And have plenty of outlets if you run electric, that way you can have many fans and a kick ass stereo for brewing time. Check out my gallery and you can see some pics of the inside of mine.
One last thing make sure you have an area out side the BrewShed to clean equipment, I will be adding a small electric water heater to mine this winter.