OK I'm not guru but I do believe that having too large an esc is a bad thing. One of the ESC's functions is to prevent you from burning out your motor. If you over prop a motor it will draw more current than it is rated to handle and since a motor is nothing but coils, metal and magnets there really isn't anything that it will do to shut it's self down when it draws too much current.

When it draws too much current the coils over heat melting the insulation on them thus causing a short circuit. Basically destroying it's self.
The ESC cuts the power to the motor when its max current rating is drawn for a given prolonged period.
Now if the max current your motor can withstand is say 20 amps, your esc is rated at 100 amps and your motor draws 50 amps because you over powered with say a large battery (too many cells) and too large of a prop the ESC will not blow or cut the power to the motor because as fast as it's concerned every thing is fine! however you're motor is way past its limits and will begin to degrade. so YA find some way to find out what you're motors max current draw is find out also what size prop/fan that thing can turn and don't go past that with your ESC.. well that is unless you like fire works!