Goto a shop you don't often go to and try them on there, not exactly a moral thing to do but how many items of clothing and shoes do women try on and never buy or buy at a different store? Heck even I do that...
Agree with jaseone... you gotta try them them first. I initially would go with the larger size (I thought it was like wearing a baseball cap, larger size felt better), but that isn't so with bike helmets. My large helmet was comfortable but I could feel it moving around as I rode and wind would somehow deflect off the helmet and enter my ears, even after adjustments. I now go with the smaller size which in my case is medium. I dial in the helmet and forget about it. Just to add, I also tried on helmets in LBS' before buying online.
Sizing can vary between manufacturers and even within models. I rode a Giro Boreas for several years in a size medium, but to wear a Pneumo I need a large. I also found I preferred the fit of Bell helmets more than Giro, but in Bell wear a large. The only way I could figure that out was wearing them.
Like you I am right between sizes. My Bell Sweep is a little more of a minimalist helmet, and in the large I have room when I wear headcovering in the winter or a cycling hat in the summer. Couldn't do that with the Giro in medium.
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