OK, I've had both size 5 and size 6  fit flops.  I would say my foot is a 5 1/2 C but I usually wear size 6.  My size 6 are large across the top and a bit loose, and my older 5's fit like a glove.  I decided on size 5, but was a bit disappointed in the quality control of the footbed,much narrower than the original, and even the though the top was soft as a glove, there was room to insert my finger between the top of my foot and the shoe, so they were still loose despite being a bit short in the footbed.  I considered returning these,but got  a tongue pad from a local shoe store to stick under the top of the shoe which helped somewhat.  I must add that I ordered an older model (new) from EBAY, and that pair fit perfectly, with a wider footbed. I also tried on a size 6 in DSW and they had the same problem.  Too large on the top. You need an extremely high instep for these to fit.  I wish they would fix the problems with QC.  I did notice these came in a wide, which I've never seen before, so maybe that footbed was more like the original.