i am thinking the best size would be 8.5x16 anything bigger will be a p.i.t.a to get around. Plus you get a lot more weight with anything bigger. Unless you have three mowers there is no need for anything bigger. You can hang every tool on the walls so no need for tons of extra room. Fuel mileage will be much better with a 16' also. This is obviously just my opinion and you know what they say about that.