All the people I know who have a size 6-8 bootie wear Large Jet fins. Including me.
Med will fit you, I'm sure of it, as long as your bootie has at minimum a soft rubber sole that wraps a little around the sides. If you have a hard rubber sole, I'll guarantee you'll fit it snugly.
I just got my Jets yesterday and tried them today. I'm a US 9 and with soft 3mm booties, I fit the Large comfortably. Couldn't get my Akona heavy duty thick soled ones in though.