YOU don't need to stretch her head out, SHE will stretch her head out. It is natural for them to stretch their neck and jaws, and they are pretty good at sizing up whatever they are going to eat. The will usually refuse if it is too big.
Here is a
video of an egg-eating snake, corns eating mice won't be stretching that far, but you get the idea of how elastic their skin is.
This is one of my favorite pictures!