If it has a prop on it now the size should appear near the base of the blades or under the prop nut.
If you do a lightly loaded wide open throttle test; get us the prop size,wot rpm and gps speed we can tell if your setup is working.
I'm coming up with either 13 or 15 pitch. Without prior running info it will be a guessing game though. maybe see if there is a prop shop near you that will let you test a prop.
2 or 3 posts down from yours there is an almost identical setup. He settled on a 17" prop and is really happy.I think his is a 2000 motor
so very likely stronger than yours.
I'm inclined to suggest a 15" prop because if a 17 is too much you should just plain not use it for an extended period.
A 15 may over rev but you can throttle back to control rpm.It will give a strong hole shot and stay on plane easier at reduced throttle.
