I bought these for a trip to Alaska and they were great.  I was able to wear my comfortable walking shoes and have these handy when it started to rain.  They pack up very small - I could fold up two pairs and put them in my backpack along with my DSLR, a mini tripod, and a small umbrella.

As for sizing - I tried three - Small, Medium, and Large.  First, they are pretty stretchy.  Just as a test I was able to force the small over a size 9 Ecco dress shoe with a relatively thick rubber sole.  On that same shoe, the medium was just right and the large was too roomy - I had about a half-inch of extra space all around.  On my trip I used them over a pair of size 9 New Balance MX10's.  They are minimal shoes but have a thick padded collar around the ankle.  The medium fit perfectly around the sole and toe area, but were a tad snug around the ankle where the padding was - it didn't constrict, but my foot did not slide in as easy as with dress shoes.

Overall, a great value and great travel rain boots.