A 10 X 9, or 10 X 11 would also be way too much prop for that engine.

To answer your question, yes, larger diameter/smaller pitch = torque, smaller diameter/larger pitch = speed.

I agree with the 10 X 5 or 10 X 6, but I think the 11 X 5 might also be an option.

(10 X 6 is the usual prop for a decent 40 size engine.)

By the way, as a general rule of thumb, when changing between diameter and pitch, increasing one by an inch usually requires decreasing the other.

(ie. 9X7 = 10X6 = 11X5) But like all rules of thumb, there's room for interpretation.
Dennis-