The boat is a scarab 300 with twin 502's and trs drives. It came with 26 pitch 4 blades, and with those, wide open full throttle was 4500 maybe 4600 rpm and 66mph. I know the motor is supposed to go to about 5100-5200 at WOT, so i got a set of 23 pitch mirage 3 blades. Just took it out on those, and 5000rpm not even wide open was only 60mph, so they're obviously too little. Do I go with a 24 or 25 now?

We could potentially Lab Finish your 25 4 blades to get to 4900. If we can do this it may offer the best of all worlds performance wise. We would need to know the model of the propellers and their condition.
Otherwise, yes on testing other props in the 24-25 inch range. We do have test propellers you can rent from us to learn what you need without having to purchase the props outright.
Here is a link to our test prop program.

http://bblades.com/propeller-test-program/

Please let us know if we can help.
Thank you,
Brett

thanks, I'll get in contact with you once I figure out why one motor decided to not to start.