I made mine 32x40x12. The wife wanted smaller, I knew I needed bigger. Mine is packed full with a truck, tractor, 5' bush hog, 5' tiller, 5' grader blade, cub cadet riding mower, and an endless supply of other necessary items. I generally use the winter to clean up, but I was on the injured list all last winter, so now I've got 2 years worth of junk collected that I need to do something with this winter.
It makes for a good excuse to spend a cold winter Saturday out there putzing around.