So, in shoes, I wear an 8w, and I have flat feet.  I ordered these slippers in a size 9.  When i first put them on, they did feel tight across the foot opening.  Tight enough that I considered sending them back, but i examined the slipper and decided that it would most likely stretch out, and since they were perfect in every other way  (cute, non-slip sole, warm and comfy cushioning, medium amount of support), I kept them.  Boy am I glad I did...I have a feeling I'm going to be a customer for life.  They have stretched out enough to be comfortable after just a few hours of wear, and I'm sure they'll relax even more with time.  This may never be a loose, sloppy slipper (and hey, sometimes I want that), but if you're looking for a warm, shoe-like slipper that you could wear to the grocery store if you had to, this is the real deal.  Love that you can pull out the insoles to wash.