shaft length

I am looking to buy a 6 HP Yamaha portable outboard motor for my Basstender 10.2 boat.
The motor is offered with 15 inch shaft length (F6AMSH) and 20 inch (F6AMLH).
How can I determine which shaft length is right for this boat? Is there something I can measure on the boat itself?
Thanks-

[boats.net]

Measure the transom height of boat to see if 15-17 or 20-22 inch high.