I ordered a size 7 medium.  I am a true size 7 in shoes.  These slipper booties fit short and very narrow.  They are offered in wide sizes but not through Amazon.  I would suggest ordering a full size up.  Also, I judge that they will never break in enough for a 1-handed pull on.  Probably you will always have to sit and pull on with 2 hands.  My slippers are on and off frequently during the evening so I want something that I can slide my foot into, lift my leg up and tug on with a few fingers.