So, you just built your first fixed gear or single speed and now you are obsessing over the question we have all asked ourselves... "What gear ratio should I use?".
And then, you start a thread asking the question that has been asked a thousand times.
If you want to calculate gear inches go here:
Sheldon Brown's Gear Inch Calculator
When you get that number calculated it might not mean a thing to you so maybe this will help.
When you buy a stock bike many companies set them up with a gearing in the high 70's which is what most folks would use if they were riding on the track, if your name is Boonen and have knees and legs of steel you might be running something in the mid to high 80's or the very low 90's.
For the rest of us mortals a gearing of 72 seems to be just about right for healthy individuals as this is low enough that you can tackle reasonable hills, ride into the wind, and still dial it up when you need to haul some ass.
If you want to go fast on a fixed gear you will need to learn how to spin efficiently at higher rpms which will make your knees, heart, and lungs very happy.
If you live in hill country or like to tour on your fixed gear a lower gearing can be preferable and I would suggest running a double stepped hub to give you a couple of gears to play with.
If you ride in the winter something in the high 50 to low 60 gear inch range will allow you to slog through some pretty nasty snow and allow you to do seated skids to keep your weight balanced and prevent wipe outs.
My fixed gear road bike runs a 72/80 and the 72 gets the lions share of use while my winter bike ran a 59, and my vintage club bike has a 69/76.
The cheapest way to change gearing is to change your rear cog and running the largest combination of chain ring and cog to get your desired gearing will make your drive more efficient and reduce drive train wear... and a 52:17 (80 gi) looks pretty badass to boot.
Messengers usually don't run monster gearing... when I did this I ran 76 gear inches in the summer and worked in a pretty flat area.