My feet are slightly too large for size M, but size L is loose enough for fins to pop off when hit by a lip, so a tether is essential, and probably a bit of padding, socks, or booties. This will be worked out. For me, sizing is rather similar to DaFin, which also pop off and require tethers. I can fit a size M with DMC Repellor, L in MS Viper, and 10-11 in POD Fins 3-PF3 Evolution, all with less risk of fins popping off.
The Leblon Flex fins are comfortable and have caused almost no fatigue in the first few days of use, which follows a summer with a lot of inactivity, including a recent, bad cold or sinus infection.
I am impressed at how easy it is to swim around the beach with the Leblon flex model. Just kicking while being on my back with perhaps a bit of backstroke was faster and more efficient than I'd been accustomed to. The Leblon has distinctly more thrust than MSViper, DaFin, or POD (right in the photo) that I've been using.
Being a large and heavy fin, the Leblon is a bit harder to walk with than the others, but the it's completely at home once in the water. The Leblon has ridges on the bottom of the foot pocket to help with traction. The drain holes, unusually on the top side, quickly clear out sand, which at our beach has a lot of shell fragments. Fins with round drainage holes clear slowly,
Catching little waves, no more that 1 meter, took a bit of re-learning. Wait farther back: It's easy to accelerate like a board rather than try to push off from the bottom as the wave is breaking. We should have 2-meter waves in a few days.
My guessing is that learning to use the Flex model to full effect will take a while. It means faster, better beach swimming. I haven't yet tried them with a camera, handplanes, bodyboards, or surf mats.