Let me start off by saying these are hands-down the SOFTEST slippers I have ever worn. And I love the sole as it has a little (just a little) traction. Now, about the size. I wear a solid 8-1/2. I don't wear socks with my slippers. Others have commented about the size running small, but since I was right in the middle and don't wear socks with them, I figured the Large (8-9) would be OK. I so wish I would have bought the XL. I didn't return them because I thought I would stretch them a little by wearing them. That's not happening and now I've worn them too long to return them. There's no way I could wear them with socks. So, I would definitely say if in doubt, even the teeniest bit of doubt, go with the LARGER size. Other than the size issue I couldn't imagine a better slipper.
***UPDATE 12-7-11***
I washed these in hot water and bleach. Took them out of the drier after 15 minutes. Shoved a size 8-1/2 pair of sandals inside the damp slippers (and I do mean shoved) and let them dry for 2 days. Now they fit perfectly.