Prop Size Question

[Dave1273]

I have been on and around boats most of my life and only recently realized, I have never had to replace a prop. The prop on my latest boat is in need of repair and the dealer said he would take care of it whenever I got it back to him but it would take a week or so to get it back. Since the boat is still new (to me) and I hate the thought of not being able to hit the lake if I get half a chance, I have decided a spare prop would be a good idea but I have no idea what size mine is and there are several options for my engine. Is there any way to tell what size prop I have so the spare will be the same? Also, I wouldn't mind dropping a few RPM's on the top end so I should probably go with a larger pitch right? What other effects will this have on performance?