Mark.
See Hackers home site:
http://www.hacker-motor.com/deutsch/

(Look under "Limited Edition, Neuer Antrieb)
This is for the new Competition motor, but as far as I understand, the recommended propeller would be similar for the standard C50.
i.e 20x13E for the 13XL, and 21x13WE or 22x12E for the 14XL.
The way I see it, is that you have to decide on which flying style you want. The 14 will fly slower, but go like a tractor on first gear uphill, while the 13 will be more groovy.
I have used the 12XL with a 19x12E, and that worked well too, so I am sure that the 13 will be good.

Magne