After several tries, it's actually a little smaller than the size of Chris Pratt's helmet. Putting your head inside is a real fight, especially with long hair.
Guys, this helmet is the Roan Toys Star Lord Helmet, I got it from KGHobby at 100€ (~120$).
This is some official pictures:
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Is not a perfect helmet but with some modifications it will be good. It is the most size accurate I've seen at this price. But at least I do not have a bubble head anymore.
Hoping to have helped you