Your toe needs to touch the front of the fin, if the boot is too large or the fin too small, the fin will be hanging on your toes and will feel like it is going to fall off when kicking. Some dive boots have wide thick soles and you might need to grind down the edge of the sole at the widest part of your foot (just a little) to allow total insertion of your foot. The sizing is often very dependent on the type of boot and sole used.
You need to try the fins on with the specific boots that you will wear. I wear size 11 shoe and can wear L jet fins with a thin boot, with minimal sole. I prefer an XL fin for the larger blade rather than the larger foot pocket, so for the XL I need a boot with a more substantial sole, (but I still might need to grind it down a little on a bench grinder.).