hello all !!! i have 3 questions/problems. please help!

1) flying Great Planes U-Can-Do 3D 90, put a saito FA80 on it. Flying slow as hell! This is my first 4 stroke engine and my biggest plane yet. Is this engine enough power for the plane? What size and range of prop should i use with this plane and this engine?

2) EVO 46 engine- sits at idle for about 30 seconds then slowly starts dropping and cuts off on its own without me touching the throttle. Many guys from the field have tuned it but i still have this problem. If i leave the glow stick on it, then it never dies even at idle. plug IS good, used plug tester and have replaced plug. i use medium plugs, if i use a hot plug will this fix my problem? seems that as long as the glow stick keeps the plug heated its fine. I dont really know being a beginner. what should i do?

3) according to prop sizes like 11x6 , 12x8, even a 13x4 ive used......a longer prop does what exactly? and the pitch does what exactly? i dont understand the performance difference of speed and torque and what it makes my plane do using certain different sizes. anyone have a simple explanation? how do i pick the best size?

anyone feel free to tag one or more of my questions, ANY help would be so greatly appreciated!!!!
thanks
-kevin