@davidfletcherphoto at least in the product description for bD's men's Half Dome, instructions are present for measuring your head and relating that to the proper size helmet. Probably for other models as well.
I am not an REI employee. i'll bet they have today off.
@davidfletcherphoto thanks for reaching out! Have you measured the circumference of your head? A quick calculation, using your hat size, looks like your head may measure just over 60cm and just under 24". If that's about right, here are a few helmet options that should fit: