My Venus is close to 9lbs and a 15x8 (break in prop) has been pretty good for that plane. Unlimited vertical, 90mph on the radar, and totally floats around when I want it to. Now I've got more power though and will be switching to 16x8. 16x6 seems to be the common prop for my 8lb-ish Yak, or 17x4w for 3D, with both the Saito 125 and YS110.

Oh yeah this was about throttle response. Sorry I forgot[&:]