Hope someone can help me with this.

I have a 16 1/2 ft Pro Craft with a 115 HP motor on it. But because we have good fishing waters in this area that are restricted to 10 HP only, I also have a 10 on it for the small waters. This 10 is souped up to a 15. I need to replace the prop on the 10 and of course I am looking for the best performance I can get out of it. I have been running the standard prop that came with the motor and it is a 3 blade. Other than that I don't know a lot about it other than it may be a 10 inch diameter. Someone told me I should go to a smaller diameter to get more rpm. I am really lost on this one, I mean I understand I can't expect a lot out of a small motor pushing a heavy bass boat but I would like to do a little better. Any suggestions at all would be appreciated.

The prop pitch and diameter should be stamped on the hub someplace.  Get those numbers and go from there but like you said, a 15hp motor pushing a full size bassboat has it's limitations performance wise.

There isnt much of a power differance between a 10hp and a 15hp.......its not really worth trying to tune it with a prop unless you have a RPM problem