An 12.25x3.75 is too much for a 45 you need at least a high torque 50 like the west for props like those, also for a trainer you would be much better of with a 11 x 5-7 or 10 x 6-8 The suggested prop is really best for a 3d type plane although originally designed for control line flying.

Cougar

P.S I would Reccomend the 11 x 7 As a good starting point.