For the setup you have, a 6x3 would probably be best. I have some Sure Starts that came on a RTF plastic that had 6x3 props installed. The props are flexible and are referred to as "rubber ducky" props by some...but they work well.

You can go with a smaller prop, but if you go smaller than a 5x4 you will have to reset the ball-socket more often, especially if you use more than 25% nitro. I like a 6x3 on mine.

George