I usually wear sizes 5-6, so I ordered a 6, but it feels too small.  It is a real tiny 6. I would recommend to go a size up.

Bought a size 8 in bottoms (usually a 6 in bathing suit bottoms) but still slides up in back.  I teach aqua and these bottoms do not work!

I'm 5'6" and a size 4. The small fits perfect.