10x6 is the "standard" starting size for a 40 engine, i have a 40FP that works well with a 9x7 and an irvine that likes an 11x4 yon have to play with the sizes a bit to find what suites the engine,plane, flying style combination.

as for the problem of climbing that has nothing to do with the prop/engine combo all flat bottom wings want to climb the faster they go, heres what i would do.

fly the plane at 1/2 throttle and trim it so it flies level.

land the plane and see where the elevator is.
if the elevator is down try adding some down thrust on the engine.
if the elevator is straight try adding a shim under the wing TE.

but as i said flat bottom wings tend to climb as they get faster