I'd try a 9-6 like CGR suggests. It really depends on which OS 25 engine you have, and what kind of plane you are trying to fly. If you have a .25LA and are trying to fly a sport plane, it may not get off the ground. A .25FX on a .40 size trainer might fly, but you will really have to fly it, it won't have power to spare.

If you have a .25FX, you could probably go to a 10-4 prop.

Good luck.