Hi, I don't want to appear rude, but why would you buy a shoe that is meant to mould itself to your feet in a size that guarantees it will flap about as suggested by ruthiebabie?
I have Crocs and they fit me so that the footbed is the exact length of my sole and the straps (should I decide to wear them on the back) are touching my heel comfortably. I have worn these in all weathers except bad winter conditions and they have never caused my any pain or blisters. this includes walking the length of Oxford St in last years July heatwave. I think I was one of the only people in comfy shoes that day!
I hope you enjoy your Crocs whatever you decide to do as they can be amazingly comfortable.