From Floops Stitch Markers:
Knitting in the round? Use the orange marker to mark beginning of row. Need the marker to hang higher or lower? These markers hold their shape quite well so you push them into a rounder chubbier shape or pinch them to be longer and svelter.
Small (fingering, sportweight) generally for needle sizes US 2 thru 4
Medium (DK, worsted, aran, light chunky) generally for needle sizes US 5 up to 10