Your question all depends on how the pipe is tuned.... pipe length determines RPM. Prop then depends on the engine's ability to turn it up to that rpm. Prop size and selection depends on the application too.

Considering the forum you are asking in here... and making an assumption for a moment that you have the MACs pre-tune pipe setup...

That setup is tuned for approximately 16,000 rpm.
I would start off with a 10x6
From there if you are looking for a bit of speed, move to an APC 9x7 and perhaps a 9x8

Find peak rpm on the ground, back off on the needle about 500-600 rpm before flight.

Bob