If you already have a fursuit made by someone else you can write to them and tell that the head is too small for you and they should make you a new one. If you made one yourself, then you should remake it, but in that case that would mean you never tried it on in the process of making... the only thing I can think of is cutting the head and just adding more fabric and foam to the point where it is big enough.
If you still don't have the head and you're about comissioning it, then you should send the maker the exact parameters of your head and tell them about your worry. And if you make the head yourself, just put it on in the process so you can be sure it will fit perfectly.
Hope I helped :3