What you want to do is to start off by guestimating what the weight of the plane will be once it is ready to fly.

I use the manufacturer's stated flying weight multiplied by a factor to get an appropriate all up weight...

e.g. if the Manufacturer states 1.6lbs I'll multiply that by 1.3 to get 2.08 lbs flying weight.

Knowing that the plane will weigh about 2.08lbs I'll use that figure to determine what type of power system I want.

Let's say we want that 2.08lb plane to be a 3D'er with unlimted vertical.

That translates into about 140 watts per lb...

So 140 w x 2.08lbs = 292watts or approximately 300 watts will be required out of the power system.

Next I choose a battery pack... say 11.1 v ( 3S LiPo )....

So 300 watts / 11.1v = 27A expected draw at full throttle.

That tells me I'll need an ESC of at LEAST 30A for this setup.

Finally I'll want to prop the plane for 300watts and 3lbs of thrust.... ThrustHP ( which is a bit optimistic relative to electrics ... ) tells me I can get 3lbs of thrust spinning a 10x5 prop at 10000 RPM and a top speed of 47mph.

A prop unloads in the air, so we want a motor that spins faster than 10000 RPM when unloaded.

So 10000 / 11.1 = 900KV * 1.24 ( to give us more RPM's unloaded ) = 1100 KV...

That tells me I'll need the following

A 30A ESC.
An 10x5E Prop
A 300 watt motor with a 1100KV rating
A 3S LiPo pack capable of sustaining 27A draw WITHOUT taxing it ( e.g. a 2200mAh 20C 11.1v LiPo battery pack ).

So by starting off with the weight of the plane and how I want the plane to perform, I get the specs I need for everything else.