My girlfriend has a small head, but the "Large" child's helmet fitted well.
I was a little worried about it being a "kids" helmet, and so whether the protection would be thinner, weaker etc. but fit seemed most important: 2 weeks ago she tested it to destruction.
She slid during a nasty traverse, dropped off a 5-10m (15-30 foot) cliff/rock band, tumbling as she went, luckily I didn't see it, but friends report she bounced stright off the top of her head...a 200m slide down a glacier later she managed to stop herself...since she was thankfully still conscious.
The helicopter guys pointed to the large crack in her helmet and raised an eyebrow: "bon casque".
Indeed - we're now in the market for a new helmet, and I am only looking at the Red ones.