I'm a 5'3" female with a tiny head, 21 5/8" circumference. I wear small/XS hats and youth-size glasses. Heck, I even have a youth-size bicycle! The medium neck warmer for 2 -12 year olds works perfect for me. It stays up over my nose if it's really cold and doesn't gape too much when worn around my neck. It also makes a perfect headband for me. If you're a normal-sized adult (versus my petite package), order the adult size (I think Simply Merino calls it large). These neck warmers are double-layered but light in actual weight which makes them warm enough for minus double-digit temperatures but so small that you can tuck them in a pocket or purse when not in use. I'm collecting them in all of the colours!