Prop size depends greatly on which plane the engine it's mounted to. I wouldn't use the same prop from a pattern style plane to a Space Walker, yet you could use the same engine on both planes. Lower pitch would be your choice.
Try 11X5 or 11X6. I think jumping to a 12X4 is too extreme, more a 3D prop for a 3D plane.

Wood, is $3.39.

http://www3.towerhobbies.com/cgi-bin...?&I=LXLG02&P=7



I fly a variety of sport planes, and like vertical performance, going up high sometimes. Some planes have pretty wide wingspans and larger control surfaces, so a higher pitch prop in both of those instances is a mistake. You can't get vertical performance with a 7 pitch prop and top speed with a 7 pitch may cause the sport plane to flutter or fatigue.
I change to a 6 pitch, then 5 pitch, if I need to to get the vertical heights I want and slow landing speeds. For every pitch you go down, effectively you can increase 1" of the prop diameter. So the load is similar from a 10X6 to a 11X5, and 12X4.