As for HOW Birkenstocks are supposed to fit, they run true to size. I realized that, since there should be anywhere from 1/8 - 1/4 th of an inch of extra space in front of your toes and behind your heel, my size is actually 40 (some styles 41).

Should you size up or down in Birkenstocks?

Q: How big should I size my Birkenstocks? ... A: Birkenstock fitting can be a very personal thing, as some wearers like to size a little big and leave extra space behind the heel and in front of the toes, while others like to size slightly smaller allowing for a snug, almost hug-like fit.

What size Birkenstocks should I get?

You can determine your Birkenstock size by adding 31 to your U.S. women's size and 33 to your U.S. men's size. Your feet should be able to move freely without your toes hitting the edge of the footbed.