According to my prop chart, the ST2000 is a 1.2ci engne and should take between a 14x6 to a 16x4. What prop works best depends on the aircraft that you have it mounted on. For a fast plane, use a higher pitch and for a slower type, use a lower pitch. I would try to keep the top rpm around 10,000 to 11,000. I think the top rpm for this engine should be about 12,500 at the most!
I hope this helped some...