At last night's TT there was talk of a nutter who uses 128" fixed. Assuming they're using the slightly-inaccurate 27" wheel diameter, that's 62 x 13. It must take a bit of winding up from a standing start. although riding fixed is actually easier with a big gear because everything happens in slow motion.
I'll be going to 48 x 14 (92" in old money) in a couple of weeks, and that's pretty headbanging.