Frog, it depends on what boots you will be wearing as much as what your shoe size is. I have size 9.5 feet, and I'm currently wearing size ML/9 ScubaPro 5mm Molded Sole Boots with my XL Jet Fins. I'm still looking for a better boot, but needed something in a pinch with a solid sole that had some depth to it, and this was all I could find in the local LDS's. This could also easily be a problem the other direction with really tall boots like rock boots.
I could've used my regular 5mm dive "bootie" boots (i.e. neoprene foot covers with a bit of a rubber sole) and fit the size L JetFins just fine, but those are about three inches shorter than the XL JetFins.
Edit: Here is the link to KidSpot's website. Good info on which combinations of boots and Jet sizes that have fit different people.
http://www.kidspot.org/personal/scuba/