If you are doing regular riding on the roads, you don't need a helmet, as they offer little to no protection against head trauma. A much better investment would be a rear-view mirror. The recent ACA study has shown that collisions from the rear are the most dangerous to cyclists; thus, it pays to keep a good eye on what's going on back there.

You'll notice that in all mature cycling environments, helmets are rarely worn; similarly, a year or so after the NYC bike share program started, in which few riders wear helmets, there has yet to be a single head injury.

So save the money you would spend on a helmet and spend one-quarter of it on a rear-view mirror. You also won't have the sizing problem