Mine is 40 by 42 with 14' side walls. Right now, I have 2 big bolts, a half ton '41 shortbox and a '27 one ton on a car trailer inside with ample room to work. It will be even better if I ever build the loft that I had planned.
From what you describe 36 by 40 sounds like a good size. Don't forget to get taller doors.
Good luck
John
