with a burton 7.5 boot you should get the smalls. They are listed up to a size 8 so you should have no problem there. How did you try them? Did you just put the boot in them or did you put the boot on your foot and then try them?
I find if my foot isn't in them, you can't crank the straps down like normal. Also, even if you're on the last hole on the straps, don't worry - they won't break. Burton tests their straps to work on every hole.
oh, and if you can find the cartel limited or regional love cartels, get those. They are better than regular cartels. Also, remember you don't need to get the est version.
If you can find the french regional cartels, let me know! or even better, the german ones (don't know if they have some)