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
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.
The idea for Floops was born out of mild to moderate frustration. Mass-produced stitch markers you find on the market today are hard and coarse. Handmade stitch markers are generally lovely but just as 'blunt'- and equally hard to maneuver. I dread coming across one in my knitting - and having to scramble clumsily over it. Soft lovely knitting, soft lovely knitting and then BANG - yuck - full stop - handfulls of plastic or metal.
They won't save the world but I think you'll enjoy using them - and looking at them!