Hi!
Go with a 11x5 or 11x6 APC. I have used both of these props on numerous trainers with the OS LA.40 -.46,and and these props are what you should use. The OS LA .40 is also a much better novice engine than the more powerful OS .46 FX or AX ball bearing engine as it provides calm steady power at lower rews ...and that is what newcomer needs...not explosive power the way a .40-.46 ball bearinged engine deliver. Here many newcomers make a misstake and think a .46 engine better...It's not!
And forget about the argument that you can always throtte down ...it's isn't so easy when you are a novice...You need steady, reliable power delivered calmly, not power delivered in an explosive way in a bucket!

Regards!
Jan K
Sweden