If you must wear a modern lid, then go for a ratchet adjustable one/rear rotation adjustment thingy. If its in a sized outer then even better.
I've a few helmets. The cheapest PASGT replica with a Flora cover fits me best, and the cover is baggy enough to allow me to pretend that a Russian bin lurks beneath it in milsim/filmsim games. The digiflora cover sits on a slightly more expensive light weight fast style helmet which is OK but crap besides a genuine FMA. I'd go FMA for a decent fit and a bit of comfort.
M1s are sized and fit accordingly, WW2 British Brody's and the like weigh a ton to play in.
Of course the true airsoft connoisseur wears a helmet of purest Vietnamese pith, preferably along with a pair of baggy cotton shorts and a Martini-Henry. Victory is assured and one can always invade the site next door and annex it for the Crown.