As others have said, a definitive guide to say which motor to go in a specific vehicle is hard to do. I have
tried to address this topic with
this page. It's not perfect, but at least gets you into the ballpark.
As usual, figure out the vehicle it is going in, the top speed you want, and the battery voltage you want to use. It's generally best to go with as high battery voltage as your ESC, charger, and wallet will allow. It's much easier to get high power with higher voltage.