You match the engines to the plane size but we tend to go a bit bigger on an engine that the plane or kit call for. Most kits and ARFs state 40 size or 60, 90, 1.20 and that gives you the main clue. You don't want to buy an engine before the plane, a .25 on a 40 size plane isn't going to work. After you get the plane you may need to decide on a four stroke or a two stroke engine.
May be a good idea to go out to a club or field and see what the other people are using and the type of planes they are flying before you go out and buy anything. It's a good idea to find someone to help you out with your choices, a good instructor is a big help. Not one of the guys from the peanut gallery, a good instructor.