Ok, they are meant to slip on, so you'd expect them to also slip off, but they are quite tricky to walk in because they do. Perhaps if the back was designed to rise up your heel a bit it might help (they're only 10-15mm high at the back). Or some kind of cinching band around the entry to grip your foot. Pretty sure I got the right size as the length is ok. They look fine which is good because big mens slippers that don't look completely dorky are hard to find.

A few months later - actually once these form to the shape of your foot they don't slip off so easily so are fine.