What's the smallest rear cog and biggest rear cog you can get?

half link chains can run much smaller cogs - as low as 8 tooth apparently

An ordinary Wippermann 1Z1 works fine on my 9t, but the issue with fixed is not getting the chain to wrap round the sprocket, it's getting it to miss the lock ring. In fact, any ordinary roller chain will, in theory, work down to 4 teeth, the exception being derailleur chains with flared inner links which prevent them from bending to 90 degrees.

8 teeth is the practical minimum for a BMX driver because you have to get a sensible sized axle through the middle of it