This is what it says about this OS 155 FS
Propeller: 16x8-12 or 17x8-12
AND
SPECS: Displacement: 1.548 cu in (25.36cc)
Bore: 1.323" (33.6mm)
Stroke: 1.126" (28.6mm)
Practical RPM: 2,000 - 10,000 RPM
Power Output: 2.6 PS, 2.56 HP @ 10,000 RPM

So does this mean I can not go down to a 6 ? damn.. and I have bunches of 6's LOL from the 120 I used to have
in the plane but never flew it with that engine. hoping this engine isn't to much power for it.

And when it is time I will have a long runway for it's first flights.