Head size 58m or Medium. Objectively, I've had 2 other comparisons, and without naming brands I found one with too big a shell (you look like a mushroom) and another in combi sizes (eg: S/M, M/L, etc) not a right fit - it was either too tight or there too much room. The GIRO Feature got the sizing for me dead-on and with a slim side profile to boot. For those worried about protection, coverage is good on the sides (as with any other lid) with an extended rear coverage (has an adjustable dial), so putting on goggles for mucky days is not a problem. One gripe is that the contact with low branches at speed leaves behind small white scratches (I'm using a black Feature). But positively, it's a reason to stick on the decals to jazz up the Feature. Comfy for an all-day trail ride, without any headache-related squeeze. Love the moto-style peak too.