Hello Hank, Welcome to Wattflyer!
Your question is a good one, lots of folks would like to know the same thing: Problem is, there is
no standard reference or formula!
Unlike glow engines, there's no "cubic inch displacement" or similar size, volume or any
measurable physical dimension that will provide a useful answer.
Generally, electric motors are measured by their power output (in watts) for a given input voltage. This input voltage of course depends on the type and number of battery cells used. Here's the four most common battery "packs" (a number of cells wired in series) and their nominal voltages;
2 LiPoly cells @ 3.7 volts per cell = 7.4 volts
7 NiMH or NiCad cells @ 1.2 volts per cell = 8.4 volts
8 NiMH or NiCad cells @ 1.2 volts per cell = 9.6 volts
3 LiPoly cells @ 3.7 volts per cell = 11.1 volts
When you look at the spec sheet for a given motor it will usually state a power output (again, in watts or occasionally, in ounces of thrust) based on one or more of these input voltage values. If it has a recommended propeller size, it will vary according to the input voltage; As the motor "sees" more input voltage it will try to turn faster, but with a large prop, it can't. As the voltage goes up, the prop size comes down. Volts = RPM, Milliamps = Duration.
Now, if we find a certain motor that produces, say, 125-150 watts of power with a 3 cell Lipoly (3s), what does that tell us?
From past experience I'd say this is a motor that would power a "400" sized aircraft very well. That would be a model about 1/2A to .10 glow size, about 3' wingspan, 250-350 square inches of wing area, and a weight (ready to fly, RTF) of between 16-24 ounces. If we used a smaller model and a small diameter, high-pitch prop, it would have a high top speed; If it was a larger model we might use a somewhat bigger, low-pitch prop and it would fly slower but have more torque, like an aerobatic model.
We could even use a gear reduction drive to turn an even larger prop (at less RPM) to power a much larger model.
If you look at the motor specs and model recommendations from a site like Hobby-Lobby.com
it will give you some idea of the range of motors for similar-sized models. Or, if you have a certain model in mind, usually the recommended motor size will power it adaquately. For hop-ups, post the model size here and you're sure to get some recommendations!
Another way to get a "feel" for motor size/output and the effect of different voltages, props, gearing and so on, it a "Motor-Calc". Here's a free one, from DMA; http://brantuas.com/ezcalc/dma1.asp
This is a pretty complicated subject and I've greatly simplified my answer, but hopefully this will get you started!