Turnigy usually has numbers like 2826 on them. The 28 is the diameter of the motor, and the 26 is how deep the can (the part that holds the magnets) is. The numbers are mostly for comparing motors. They don't really tell you the performance of them. Obviously, the bigger the motor is, the more powerful it is in general. I'm sure you know what KV is and what it means. It's how fast the motor spins.
The best place to start is to pick the prop and KV you want. A static thrust calculator like this one: [link]personal.osi.hu/fuzesisz/strc_eng/[/link] : will help to get an estimate at how much pulling power the motor will have with any given prop. Then find a motor that has the KV you want and can handle the prop you want. Usually going with what other people have said about the motor on the hobby store's website is the best info.
It helps to have an estimation of the total flying weight of your plane so you know about how much thrust you want. You don't need more thrust than what the plane weighs. I don't remember what percentage is the minimum is. But you can always use less throttle, so it's better to over power than to under power.
EDIT: I should note that the calculator I linked asks for the RPMs. You have to multiple the KV of the motor by the volts of the battery you'd use to get the RPM.