I think that like saddle fit, helmet fit is clearly a very individual thing.
This helmet, for me, is the most uncomfortable helmet I've ever tried. I have a 58cm noggin and the helmet is a size medium. For me, the foam at the front of the helmet instantly digs into my forehead and is instantly painful, even having it on for a few minutes leaves impressions in my forehead. It's not so much that the helmet is too small, I can stuff additional padding in there and it helps, but no extra padding is provided (what's there is very minimal ~1-2mm thick strips). I've tried contacting Giro to see if they have any extra padding kits etc or can offer suggestions but they did not reply. So a big thumbs down from me on this one.
I would recommend trying one first hand in a store before placing an order, if you have anyone in your area that stocks these. If your head has the same problem as mine does with this helmet you will know it instantly!