Selecting the correct prop in electric requires that you have an amp meter. First, you can get a good estimate of the correct prop for your motor by using one of the many prop calcluator programs on many websites, or Motocalc. Next, buy that prop, and maybe one above and one below. Then, insert your ampmeter and test the current draw on a bench. At wide open throttle, you should not exceed the motor or ESC recommended amperage. Thus, if the motor should not exceed 45 amps at WOT, then this should be the case on the test bench. If your draw is too low, then move to the next higher prop. If too high, then move down a prop. If you increase prop diameter, you get better torque, if you increase pitch you get better speed, so you can increase or decrease either diameter or pitch to get the current draw right depending if you prefer torque or speed. Then, what you get in the air may differ because the prop unloads in the air. Thus, you may want to get a logging amp meter that you can leave in the plane when you fly it and then read the results when on the ground. I use the logger from http://www.eagletreesystems.com/ to fine tune the correct prop. With nitro, you do not have to do all of this. With electric it is essential. If you draw too much current, you will smoke your motor and your ESC quickly. Also, pay attention to the "C" rating of your lipo. That sets the limit on what current the lipo can dispense. If you have a 3000 mah lipo with a 30C continuous rating, you multiply 3 amps times 30 to get the maximum of 90 amps continuous current draw. So the entire chain from lipo, ESC and then motor amp rating must not be exceeded by the prop selected. At the end, current times voltage equals watts at WOT. You want between 75 to 175 watts per pound depending if you want docile or aggressive aerobatics. I keep a box of various props handy to prop up or down when needed. If you do not have enough power you can also increase voltage by going to a 4S, 5S or 6S or so lipo system, keeping the current constant, provided your ESC and motor can deal with the higher voltage. Remember that above a 12S you are above 50 volts. At 50 volts you exceed skin resistance and can get a nasty shock from your power, so it is unwise to play with more than a 12S system to get power up. However, now there are guys who have in excess of 7000 watt power systems in RC planes using 12S lipos and 200 amp ESC/Motor combinations. That could fly a 70 pound plane! Many times if you visit a forum for someone who already has your model, and your power system, the prop they use is a good starting point, assuming they put an amp meter on it to get it dialed in right.