A.
anwered by: canteau
You will need a hub that accepts metric threaded freewheel. Commonly these are only found on BMX "Flip-Flop" hubs which are being phased out with the popularity of cassette drivers, which allow the use of gears all the way down to 9t.
If you need larger gearing on a road or mountain bike it is better to use a larger front chainring which will give you better chain wrap and be less likely to come off. Surly has a good blog post of single speed gearing which can be found here:
http://surlybikes.com/info_hole/spew/spew_single-speed_gearing_101