I'm guessing now but have you ever built or assembled a plane before?? Now you have me worried about the engine too. Have you ever tuned one? You buy a mount for a .91. If it's a great planes it will state on the package the engine range the mount fits. If it's an aluminum mount then it will state the size. Horizon sells some good ones. There is a prop chart sticky in building or beginners. A lot depends on how you want the plane to fly. I like longer props with a smaller pitch. I had two planes that went the other way to slow them down but I never gave it a lot of thought wondering why they broke the rule. On the 4* I would be testing a 15X4 or 6 right away but prop testing takes me a complete day until I find the prop I like. The engine has a working RPM range and you don't ever want to go over that and if you over prop too much it will lug it down. Just keep it in the working RPM range and you will be OK.