The propeller determines how much current the motor attempts to draw from the ESC and battery.

So it is always best to use what the plane came with ( in size ) if you are using the stock setup.

Measure the tip to tip distance of the existing prop and you'll know the diameter... you'll be safe with a 6 pitch prop, though you could also try a 7 pitch prop for more speed.

Make sure you get an ELECTRIC prop e.g. an APC "E" prop.