Most of us have been there. Know that you are not alone. I have one way to big & one just a little to small. Third time is the charm. lol
Like Mirage said, go check out those threads. They are made to help with those problems & more. Read all you can first then try again.
You will have to learn to set it up for yourself after you read & watch the threads on how to scale it. You are the only one that can get the scale right for your body.
Just think of the first as a test piece. That way any mistakes are on it & not your final helm.
It sucks but everyones head is a little different. I myself have a big head. lol. I know it & work with it.
Good luck