I just put this together so you all can get a better idea of how the sizing goes (a more refined version will be on the website shortly).
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This is how the complete boots are specified from our side and I would use it as a guide for selecting your strap. As you can see, there is naturally overlap on the "M" straps (if you are a size 27 Hawx boot, you can wear either size Dual Strap because they achieve the exact same thing). Sizing will depend firstly on your boot size (larger boots have larger diameter cuffs) and secondarily on your calf size. If you have a 27 Hawx with a skinny leg, you'll be fine with the S/M. If you have a 27 Hawx with an "athletic calf", go with the M/L and it can accommodate a larger leg if needed.
Even easier- If you are 26 or smaller, get the S/M. If you are size 27 or bigger, get a M/L strap. That will more or less apply to both Hawx & Redster versions.