I have found that using a bit of wool as a stitch marker can be frustrating as it can get caught up in the stitches. I got some metal ones free with a magazine and I found these really good. I have 4 of these blue stars but with lots of WIPS and the majority either in the round or shawls I found I needed more.
I got some 2″brass eye pins and some beads,
threaded the beads onto the pins
These markers are useful for any needle up to a 6.5mm. When I need bigger ones I will just use a bigger knitting needle to bend the pin.