I have a really small head (I wear kid's glasses and hats), but I have always worn an adult small Bell helmet. Make sure you get one with the ratchet dial in the back. They cost more, but it allows you to dial in the fit, to fit different conditions, without adjusting the straps. This pretty much eliminates the sliding around thing. So, if your head swells in the heat, or you wear a head cover/hat in the cooler weather, you can easily change the fit.
The model i like has been discontinued, but we bought a couple on sale, so I have 2 and switch between them. It's a mountain bike helmet, but I cut the visor off. I like the ventilation in this model.