I tried on a helmet and I'm pretty sure I'm a size medium. My question is that when trying to slide my glasses through the open shield, I was able to worm one finger from the side to my ear to help adjust the glasses to my ear. Is it ok that I was able to flatten my hand against the side of my neck and work a finger or to up to my ear? Hopefully that's not a sign that it's too big. I could fit into a small but it did feel somewhat tight (not really any room to wiggle the helmet). Plus it wasn't pleasant at all getting it on. At first, I didn't even think it would be possible to get it on, but with some determination, I was able to (my ears did hurt getting it on). With a medium, I could wiggle it just a tad, but it doesn't shake freely when I shake my head and doesn't come off when I tuck in my chin and try to push upwards from the back.