i would first ask the reason why?
what kind of bike do you have? do you know what kind of wheelset/rims?
my friend has an SE singlespeed that came with 700x32s....im just going to assume you got a stock bike like that with 700x32s....so chances are you could go down to 700x28s, or some 700x25s, but im thinking 23s would push it.