Most 50cc engines I've seen on aerobats prefer 22/8 to 24/8. Menz seems best, although don't count out Mejzlik carbon blades for a bit more money. If you're on a light budget, I'd recommend the Pro Zinger brand props. NOT THE REGULAR ZINGERS!!! Regular Zingers are very poor props.

Or you could buy a 22/8 PAC prop for about $175 hehe...