You have it right Sunbeam.
In the smaller motors, 15" is short 20" is long and 25" is extra long
In the larger motors, 20" is short, 25" is long and 30" is extra long.
So to get them the same to hang on the transom,
you need a short main (20") and a long kicker (20").
Because when using the kicker, you are not on plane, a clean source of water flow is not as critical, especially if you use a mounting bracket that gives you a bit of set back or Garelick style bracket that drops the motor when you use it.
I have a 15" 5hp on my 22 mounted on a fixed bracket with some setback and even though the cavitation plate is a few inches above the bottom of the boat is still works well.
If you are going to mount it directly on the transom I would think a 20" would work best. If you are using a bracket that allows some flexibiltiy in mounting height and some setback. you could use a short 15".
The short motor is a bit lighter, easier to store and move about and works better on small boats. There seem to be a bit more of them available and they would be easier to sell if you want to get rid of it and you could use it on a dingy, square stern canoe or jon boat.
Good Luck
