Ezee,

According to the MAN May, 2003 .60 engines shootout, most muffler equipped .60 engines produce their maximum power output at around 13,000 RPM. Your ST is no different (unless it is equipped with a high RPM tuned pipe, or is in open-exhaust guise...).

So it should use a prop, that it could spin at 11,500 RPM, more or less.
An 11x8, 11.5x7, 12x6, 12.5x5, or a 13x4, should do the job.

In a sport plane at speed, the prop unloads about 1,500 RPM, so the engine will make its maximum horsepower when it is flying your plane.

Using an 11x7, your engine will be close to maximum power on the ground and will be beyond it efficient RPM range, gasping for air, in high speed flight.

It will fly the plane more slowly too.