What size prop should be 20 foot pontoon boat with a 50 hp mercury outboard?

I have a 19' Crestline with a Mercury 50 ELPTO that has a 12X101/2 Turningpoint prop. Max speed with (2) aboard is 23 MPH by GPS @ 5600 RPM in ideal conditions. I crusie at 4000 RPM at about 13.5 MPH burning 2.5 gallons per hour. Max speed drops to 20 MPH with (5) aboard and will crusie at 12.5 MPH at 4000.