I found this at R/C Proving Grounds (http://www.geistware.com/rcmodeling/...ower/index.htm)

How are motor sizes specified?
Motors are traditionally specified by a system which attempts to equate them to wet engines. There are significant problems with this, but they probably aren't of concern to beginners. An "05" motor takes a six or seven cell battery and puts out 75 to 120 watts, and so on up to a "60" which takes 28 cells and puts out 1200 watts. Incidentally, there are about 750 watts in a horsepower.
The actual power output for a given voltage (number of cells) depends on the load. Unlike wet engines, electric motors put out more power with more load. If you don't like the performance you get from your plane, you can try a bigger propellor, up to a point. More power, of course, means less run time.
In the ideal world, motors would be specified by the total power they are capable of supporting and by the number of cells (or voltage) with which that power is produced.

Maybe you can make more sense of this......