Shortshaft models fit boats with 15" transoms and longshaft models fit 20" transoms. Many smaller older longshafts are shortshafts with extension kits either as accesoriy or factory installed. Most modern models are completely either/or.
you'd think so , but they're all over the place, and so are outboards. 15" or 20" is typical but not carved in stone.
I think it's my Merc 9.8 short that measures about 18"
My one cartopper boat's transom is 17.5"