make it as big as you can, you won't regret it..
I have 3 buildings a 40 X 90' for cars, trailers and big stuff, A 32 X 64 shop for projects (Cars, gas pumps,etc) sandblast cabinets, powder ovens, hoist, a 10'x 32' storage area and all walls are covered with signs. A 24 x 28 building covered in signs that I use as a clean room for assembly of projects and I bought a 53' semi trailer for storing cas pumps.
The more room the more stuff/projects you seem to find and keep!!!
Plus I have a "Pure station" 1 or 2 stall building in my near future.