Matthias would be my choice, and not shortened to Matt. We don’t shorten names in our family, because we like the names we chose as they are.
If you will ever notice, children’s names do not get shortened nearly as much as they used to. The full first name is used in address and in referring to oneself. Don’t know why this is.
And in our extended family, for some reason, we tend to use entire names when referring to another relative — “John Smith”, “Mary Brown”, and so on. I don’t know why we do that, we just always have. For one thing, the family is so large that there would be several “Johns” or “Marys”, and it helps to clarify of whom one is speaking. I have noticed that among Irish Travelers in this country, they will use descriptive names — “Big Joe’s Katie”, “Pete Man’s Sarah”, and so on. Interesting.