Actually, you can use any prop that will allow your SuperTigre 90 to turn about 12,000 RPM when on the ground. Any prop that will allow the engine to turn that RPM and fly the model the way you want it to fly is the correct propeller. Running a prop that bogs the engine down too much only causes heat and extra wear.

We normally use a 12 x 8 prop for testing, so you could logicially go to a 13 x 7 or 14 x 6. Getting up to a 15-incher may cause too much load, even with smaller pitches because of the sheer weight of the bigger props.

The SuperTigre S-90, S-90K, and G-90 are all ported to run at higher RPM than other engines of similar size. 12K static RPM is a good number to shoot for when leaned to just rich of peak RPM.

For scale models, use the propeller that flies the airplane the best, and ignore scale appearance. For scale appearance, substitute a display propeller when you're not flying. When the engine's running, the prop's pretty much invisible, especially once the model's off the ground.