Last tiime out we tried several props on my buddies Nexstar. The 11x5 that came with it was ok, but very fast and hard to slow down.
12x4 was much better but the pull seemed a little week.
12x5 was just about right
Just for informative purposes I put a 11x3 on it to show my buddy how a prop can make such a differance. That little engine was screaming but the plane was pokin along. Acted very tail heavy even though it is actually a bit nose heavy.

So my $.02 worth, 12x5 or the stock 11x5 if you don't mind the speed.