I have gotten blisters with my crocs, while I wasn't wearing socks. My crocs are large enough. I think it's just because my foot is somewhat narrow, and my foot tends to slide around a little in the shoe. It makes for a very comfortable shoe, but sometimes I will get a blister on the bottom of my foot at the center of my heel, almost where my heel goes up onto the back of my foot. I still wear my crocs even after that though, but just put a bandaid on the blister and wear very short footie type socks for a couple of days. I've just decided that any kind of enclosed shoe is going to give me blisters if I walk in them enough. I can't wear flipflops for that much walking and at least with the Crocs, my legs don't hurt at the end of the day.
I'm always on the lookout for a better option and have spent a small fortune of other shoes just for WDW, but so far, my Crocs are still the most comfortable for me.