I prefer to work from an estimate of the total weight of the plane plus a factor for bigger batteries, then use the resulting weight to find my watts/lb.

Then take watts/lb to determine the prop and motor given what beau0090_99 posted.

You are advised to ALWAYS leave a 20% "overhead" available... in other words never run your electronics, ESC, Motor, etc. to 100% of their capacity.

Your batteries will thank you for it too.