It depends on whether a ball bearing or bushing engine and what your application is. If an OS engine the instructions give some good guidance on propellors to use for various applications. I have a 46FX and use 11x6 propellors for funflying on large trainers but you can go 11x7 just as well. I have a LA46 and I fly it on a combat spad with 10x6 or 9x7 propellors no problem. check your manual