Here's the sizing explained on the Crocs website. Plus if you go to the site there is a PDF you can download and print that you can use for sizing.
DBF and I went to try on some before we went on our trip. Their traditional Beach style fits me weird. I generally wear a woman's size 6.5 or 7 but I'm between sizes with the Crocs. The Small is perfect if I don't use the strap behind my heel. However if I flip the strap then it's too tight. If I go up to a size Medium, my feet swim in the shoe as there is a HUGE difference between the small & medium. I ended up not buying them but am hopeful to get a pair someday.