What Sue says is good. If no matter what you do with needle size and bind off your neck hole is still too small you might have to go back to where you have a few more stitches. If it comes to that and you need help the name of the pattern and where it could be found would be helpful.
A bind off I have started using is Jeny's Surprisingly Stretchy Bind Off. I found instructions for how to do it with a Google search. I just used it on a rolled neck for a baby sweater and they have big heads.