Almost everybody who regrets a purchase in this category bought too small. Here is how to size it properly the first time.
Measure the biggest thing going in, then add walking room on the side you need to reach it from. A riding mower needs clearance to turn, not just to fit.
A building fills up faster than anyone expects. If you are between two sizes, the larger one usually costs less per square foot and gets used.
Use the Footprint Finder on any page. It only offers combinations we genuinely stock, and it draws the footprint to scale against a parked car so you can judge it properly.