You're kinda working backwards. Here is a breakdown of how I size a motor/esc/prop combo (doing a glow conversion mostly)

Look at the size prop you can put on the plane and choose one (let's say it is a .46 conversion). 12x6 prop (just a swag)
Look at the weight of the plane and the type of flying. 75W/lb for trainers up to about 150-200W/lb for 3D.
A decent outrunner motor should be limited to about 3 watts per gram of weight of the motor.

Let's use a 7 pound trainer now as an example. 7lbs at 75W/lb is 525W. At 3 grams per watt we're looking at a motor of at least 175 grams. Ok, this will get us in the ballpark.

Now, this is where the iterative process starts. Looking into the different motors available (I shop Hobbycity.com a lot) You kinda have to fit the motor to the prop and back and forth. It helps to have a good database to start with too. I use Drivecalc, which is freeware and is powerful. Here is what I do:

knowing that I have a 12x6 prop and I want to get about 525 watts of power into the prop I need to know how fast that prop will need to spin. I have a program that has the prop constants in it called ThrustHP. Using this and knowing I have the 12x6 and 525 watts, I can convert for HP and get about .6962 HP. Knowing this I can play with the RPM number to get the same power. I see that 9250 RPM will give me about the power I want.

OK, now that I know that I need about 9250 RPM at load, I can find out what my no load speed should be. At about 75-80% of no-load most brushless outrunners make the most efficiency, so if I take 9250/.8 I get about 11,500 RPM.

I would use this information and the motors that I typically shop to find out what the range of Kv is for the motor s in the 175-200g size range. Knowing that I shop HC, most of these motors are between 900 and 1100 kv (looking at the Turnigy SK35-48). So if I use one (1100 kv) and take 11,500/1100 I get 10.45 Volts, and if I take 11500/900 I get 12.7V. So a good nominal battery might be a 3S lipo. and because it is nominally 11.1 v, I would want to choose the 1100kv motor so I can get at least 11,500 RPM nominally on the prop.

OK, now we have a prop, motor, and battery type, what size do we need? Well, at %80 efficiency getting 525 Watts out of the motor, we would be putting about 650 Watts into the motor. If we have a battery that is 11.1v, then we would be pulling about 60 amps peak. This tells us that we need a battery that at peak can provide 60A, and also an ESC that can handle 60A peak.

How big of a battery do I need? Well, common ratings are 20C continuous and 30C peak, so if you had a 2200 mah battery (which is very common), then at continuous it should be able to supply 20 * 2200 or 44A cont. and 66A peak. I always urge you to add about 15-20 percent overhead. 2600 mah batteries and 3300 mah batteries are getting more common and they will allow for higher current drains and will give you better battery life if you are running that high of current.

This is a pretty comprehensive overview, but these are the things I run through when doing an electric conversion. If you want help on your specific project, feel free to PM me.
Best Regards,
Curtis