question about outboards

I am in the process of building a duck boat that will be my utility boat, I have redesigned so I can also use it for fishing.
The transom on the boat is 17", which means that I will be needing a short shaft motor.
My question is, how do the measure the shaft length?
I am assuming that it is measured from the mounting bracket to the center of the prop?
What is the length of a short shaft motor and how is it measured?

Thanks in advance for your help.

short shaft is 15"
long shaft is 20"
there is a xxlong that is 30" , don't see too many of these, sailboats use them
there is 5" between sizes

To measure a motor, you measure from where the transom bracket sits on the transom to slightly above the cavitation plate. I just used to measure to cavitation plate normally within less than 1/2 a inch. it either is 15 or 20 & a fraction.