These slippers are the most comfortable footwear I've ever owned!  Since many customers seem to be confused as to which size they should order, here's a bit of advice for you:
I live close to the Union Square Shopping District here in San Francisco.  I walked over to the Downtown SF UGG Store and tried on the Ascot Slipper with Store Manager assistance.  My foot measures EXACTLY size 9 D (M), so I tried on both a size 9 D (M) and a size 10 D (M).  I can see why many customers are saying to buy one size up because the abundant amount of UGG PURE wool made my true size 9 D (M) feel quite snug.  I noticed that the size 10 D (M) felt like it fit better at first, but after walking around the store (with regular cotton socks on) I noticed that the heal started to slip just a bit.  The Store Manager said the vast majority of the UGG Ascot Slippers she sells feel tight at first, but "break in" to feel quite normal after several days of wear.  To avoid excessive returns, she ALWAYS tells her customers to order their exact street shoe size, NOT one size up. So, I decided to purchase the size 9 D (M) from Amazon.com and I'm SO GLAD I didn't order the larger size!  I plan on wearing my slippers with no socks, and it's important to know that I have normal (to slightly narrow) width feet.  If you plan on wearing thick socks, or have wider than normal feet then you might consider ordering a size up, since half sizes aren't available (or possibly a wider size if you have wider than normal feet).  Everyone's feet are unique, so you will have to decide if it makes sense to go up a size as several customers have suggested.  I'm glad I didn't order a size up though, because my heal would be slipping by now if I had.
I hope this sizing information helps!  Give yourself the gift of comfort with these awesome slippers, they are well made, toasty warm and great for wearing around the house on chilly days.