Directly from OS specs on this engine:
SPECS: Displacement: .545 cu in (8.93cc)
Bore: .906" (23mm)
Stroke: .846" (21.5mm)
Practical RPM: 2,000-17,000
Power Output: 1.75 ps/16,000 RPM
Weight: 14.29oz (405g) w/o muffler
18.52oz (525g) w/ muffler
Recommended Props: 12x7-8, 13x6-7
You can never go wrong staying within what the manufacturer recommends.
Ken