Clearly the world needed Crocs. When they first appeared (15 years ago?) they became the standard indoor shoe, with enough arch support to be worn for short walks, even driving. They also make a good water shoe.
These days I don't think they hold up as well as they used to (Chinese manufacture?), and the sizing is rather mysterious. I wear a (women's) 9 shoe and got a 8/10 M/W that seems just right with socks. Probably best to see what size the pair you have is (it's stamped on the bottom) and order accordingly. We must have at least 8 pairs of Croc between my husband and me (sometimes we end up wearing each others').