I’m planning on buying a new helmet - most likely the giro pneumo. So far from what I’ve seen a “large” head is 23.5 inches and up. I measured mine… it’s 23 inches. In buying a helmet should I account for heat and swelling? Should I get the large helmet or go with the medium? I’m ordering it online (no stores around here sell them) so I won’t be able to try it on…
You could always order both, and return the one that doesn’t fit 
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I wear a medium Eclipse and I’m borderline at 22.25 inches. I would go for the large.
It might be worth while to drive 20 or so miles to a store and try one on. Giro helmets don’t fit everyone the same. It would be terrible to find that it just isn’t the right fit for you after you dropped $110 on a helmet. Performancebike.com will let you return items that don’t fit for just the shipping charge. I’m sure other stores would too.
Good luck
jaretj