Info needed to compute proper prop size?

[Kikwear]

This weekend, I'll be out on the river. I'd love to see if my current prop (don't have the numbers in front of me) is the best for my setup. What do I need to get the right info so I can call a prop company to get the best available?

-GPS for speed
-WOT RPM
-Altitude
-Present prop numbers

[Fish boy]

Gear ratio would also be helpful.