You have to create a false pair of legs. You have to make it so the player can't see his own player model besides his arms, and then have him see a false pair of legs that are scaled down to the right size.
Just take some pictures with a camera strapped to your face and see the limits of how far your neck can stretch, and see the full body motion required to see your legs, and incorporate that into your design.
This was the method used to create all the arms and legs for Mirror's Edge. There is also a concept on gmod which you can see http://www.garrysmod.org/downloads/?a=view&id=110052