For the Shifts, I wonder if they are imagining people using boots when they say to mens size 7. They work well for me at size 10 and I could see size 11 working. There is some wiggle room in the fore/aft position of your foot depending on how you tighten the straps.
For float, it’s so conditions dependant that it’s hard to say. You’d likely want to be under 100 lbs if you were using these in soft winter snow, but they can hold >200 lbs when it’s more consolidated, so it really depends on the conditions.