I actually owned a pair of Vortex V12's for my first fins, they're tough a nails, and heavy as expected. I swapped out for the apollo's without the spring strap, then later upgraded to c-straps.
I was on the low end of the med. Strap size range, but it fit well. So to answer your question, sizing isn't a huge concern since you can drill the holes further or closer a needed. As long as you're within your size range, it should work.
You'll need to remove your manual straps,
Cut off nubs on your fins,
Measure spacing before drilling holes for new straps,
Space the holes so that there's about 3/4in of stretch in the spring when it's around your ankle.
Drill holes, fasten springs to fins- recommend loc-tite on screws.
Jane H Thanks EUN! This is very helpful.