These are great looking and comfortable slippers, AND ARE ROOMY ENOUGH FOR WIDE WIDTH FEET !  But like so many slippers they don't have a clue about sizing !  Shoe-wise I wear a 10.5/wide, so I ordered a 12M as was recommended to go larger.  My heel sill sits on the back edge of the slipper. Its close enough, and they are still comfortable, so I kept them.  Been wearing them daily for 2 months and these are the first slippers that I can really say I love how they fit !
For anything close to my size (10.5/wide), I'd say go at least 2" larger and you will be happy with them.