Short Shaft 15"    Long Shaft 20"
Please Note:
 
Information gathered on this page was copied from various sources to help you make informed decision in measuring your boat transom. We greatly appreciate their information. 
 
Small Jon Boat
Sailboats
Pontoons
Small Boats that have 18"-21" Transom
Sailboats with movable brackets
Small Jon Boats
Small Sailboats
Inflatables
Gheenoes
Canoes
Small Boats that have a 15"-17" Transom
 
The information below was gathered from many informative sources but mostly from Trailer Sailor Articles & Reviews.
 
The "Outboard Boater's Handbook" suggests that a good rule of thumb for motors is 1HP for every 100 pounds of boat to be moved.