The round stitch ring markers come in small (10 count) and large (10 count) sizes.  The rings are inserted on the knitting needle to mark as a guide for increasing and decreasing a stitch.  Also, for marking at the beginning and ending of the round.  Come in colors of teal and orange.
 
For needle sizes
Small (sizes 11-15 needles)
Large (sizes 17-35 needles)