There were brushed electric motors that were called 300, 400, etc., and the numbers reflected a arbitrary scale of increasing levels of power. There was no real logic to that but the names came forward to brushless motors so that people that knew brushed motors could get a comparable motor.
Metric measurements of brushless motors, usually diameter and length in that order . Sometimes they are measurements of the stator inside the motor, other time they are the O.D. and length (not including the shaft) of the outside of a brushless outrunner. No standardized system for it, you just become familiar with them.
Believe me, the people that selling them do not mind at all if it takes you several purchases to ge a motor that is right for you...
Some rules of thumb will help.
Power in Watts = 2000 x I.C. motor displacement in cubic inches. So a .15 motor should be replaced by a 300 Watt motor (2000 x .15 = 300).
Watts by motor weight - Average quality motors will produce 3 Watts of continuous power per gram of motor weight. So a 92 gram motor is an 276 Watt motor.
The Kv rating on a motor is a key criteria when picking motors and the prop and battery voltage are what determine the Kv you need. It just has to be learned for the most part. But you can start by looking up the prop you want to use at FlyBrushless.com and looking at the thrust/RPM graph. And