I have tried the largest helmet I can find the bern 2xl 3xl wont fit. The problem I am having is the width of the helmets are not wide enough. I have a big head.
Helmets.org is a nonprofit organization that compiles lists of helmets that fit specific needs, including a list of helmets that fit particularly large heads. Their website ToS bars verbatim copying (even though data like that is probably not protectable), so all I can do here is link.