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I'd try to get one from a store if you don't know the correct size. When I bought mine (different brand), the size was a lot different than I would have expected, so I was glad I bought it from the store.
I always want to be shure it does not move around on my head when it fastened on me and it is not to tight on my forehead that it is uncomfortable. But mine doesnt ever move from my head and neck restraint they make us wear now.