This advise is for the homemade springs, not the commercial ones, dont know what size the commercial small, medium, large are for actual sizing. Have you searched online for data from the manufacturer of the straps you are considering?
I just made my own spring straps following advice from the DIY section. I have size 10.5 feet, use Mares Quattros and i used the whole of the 11" springs from McMaster Carr, check out the thread in the DIY section for part numbers etc. You would need to measure from one post to the other around the back of your heel with your boot on and in the fin. The posts are where you attach your S hooks or shackles to keep the spring in place. I clipped on side of the spring on and stretched the spring around my heel to feel if it were loose or tight that way before trying to adjust the length - but they felt ok like that and so i kept them full length.