I am flying a 40 sized 3D plane with a magnum 4 stroke and love how it flies.
Right now it has a 14X4W apc prop.
What size prop would you guys recomend?

Go with the APC 15x4W ... I got it on my TT91 4C and its nice. I still get about 10K RPM with the prop. The 14x4W is a just a tad small for the Magnum 91, its best suited for the SA82 if you run 30% nitro.

I run a 16 X 4 APC on my 91, but it is a YS. Still, the engilne winds right up with it with no indication of being oversized. You might give this size a try. The 14 X 4 or 15 X 4 is good for the YS 63.

15x4 should be just about right...

I tried a 14X6MA today and I liked it a lot better. It still had tons of torque but would pull out of a torque roll a little faster.

Whats strange is that I was getting almost identical RPM's out of the 2 props. Both around 9500 on 10%. Its a new engine and is still being run rich so I think by the time it has a few gallons through it I hope to get a good 10000 (which is considerably higher than I thought I would get!!!).

I will try the 15X4 next.