DearfoamsChenille A-Line Women's Slipper

Q: how is sizeing work. what is a size 6 1/2 ?

I wear a 9.5 normally, so ordered a large. Sadly it fits more like an 11, falling off my foot. I have to wear at least 2 thick socks, but it is winter so I am ok. I should have returned them but didn't because I believed if I washed them in hot water they would shrink around the top. They didn't. I miss the ones I have ordered for 10 years, they always fit just right (also dearfoams, but not made any more evidently)