Last December I managed to pull a large mole off my back going through the squeeze into Heaven, in White River, and that's polished smooth flowstone - at least the bottom is. It was hanging on by a thread when I got back to the hut - I didn't even know I'd done it, but luckily someone spotted the blood on my back when I got out of the shower. I got them to plaster it back on, and then had to wait five days before I could get it examined at the doctor's, what with their busy diaries and everything. It's gone now, as I had it removed a couple of weeks later, explaining to the doctor (quite seriously), that I had some tight squeezes to get through, and it wasn't worth the risk of doing it again. He did look at me a bit funny, I must admit.
But I literally know my limit now, and it's about six inches. :-[