Whatever you do build it a little bigger. If you think you need 30x50 then build a 40x60. They fill up fast. If you build too small then your options become very limited and undesirable (lean to, second small shed, etc.) Just do it right the first time and thank me later. Nobody ever said that their pole barn was too big. If you ever have plans of adding a small living space, bathroom, etc you will run out of space especially fast.
I am very fortunate that my home has an attached 4 car garage (30'x50'). With my TJ, 1/2 ton pickup, wife's suburban, and Ford Escape there is not hardly enough space to comfortably do maintenance/projects without pulling vehicles outside. Then stuff lawn mowers, ATV's, and toolboxes, work bench, air compressor, and kids toys there's not shit for room. If it had another 5' on each end and an extra 10' wide I'm doubtful I'd ever need anything else. One of these days I'll be putting up 54'x72' is the long term plan tho.