Originally Posted by TimArchy

There is some argument as to the effects of different combinations which create the same ratio (say 3:1 - 42x14, 45x15, 48x16, etc..). Traditionally, it is thought that a smaller ring/cog would give better acceleration while a larger ring/cog combo would allow you to "stay on top" of your gear more easily once you were at speed. Arguments were based both on empirical evidence gained from the observations of riders and on the idea that the relative difference between the diameter of the ring and the length of the crank somehow effects the amount of power that is translated into the drivetrain. This kind of thinking eventually led, in part, to the creation of the Shimano 10 Track group, which was based around a 10mm pitch chain instead of a half-inch pitch, which is the current standard. It allowed for smaller chainrings and cogs which contained the same amount of teeth as normal. Although it was marginally lighter than the competition, it didn't catch on.

More modern thinking has come to the conclusion that whatever benefits are gained by the decreased diameter of the chainring/cog is mostly canceled out by all the sources of friction in the system and generally not worth the bother.

A good idea is to keep your cog size between 15t and 18t. These are the easiest cogs to find when you need a replacement (16 being the most common). The same can be said for freewheels (with 16 and 18 being common). From there, choose whatever chainring you feel is good for you. Something like 45t will give you a lot of room to experiment with different gearings while giving you the most skid patches, should you decide to use skipping and skidding as a method of braking. It will also keep your gearing on the low end for all the hills.

Most SS/fixed frames with track fork ends are able to accommodate up to a two tooth difference in the rear cog without having to alter the chain length. This allows you to put a larger gear on one side and a smaller gear on the other. A common trick is to have a 16t fixed cog and an 18t freewheel. The freewheel can be smaller because you don't have to worry about spinning downhill.

Does this answer your questions? It is good to remember that bicycles have been around for a long time. And fixed/SS bikes were the first. Anything that anyone can think of to greatly improve the quality and comfort of the ride or the usefulness of the bike has already been tried by someone. If it works, it is generally picked up by the majority of the community. If it doesn't work, it isn't. Hence, you can learn just about everything you need to know about bike set-up (not including fit) by looking at the bikes around you. It becomes pretty easy to see which bikes are practical and which ones are impractical simply for the sake of being different. At the risk of sounding conformist, the more normal the bike looks, the better it is at performing its job.