Perhaps Sheldon spoke of this, yep....
http://sheldonbrown.com/fixed.html
Big or Small?
nce you have decided on your gear ratio, then there's the question of which of several different equivalent sprocket/chainring combinations to use.
For instance, 36/12, 39/13, 42/14 45/15 48/16 all give the same 3:1 ratio. Which to choose?
Bigger Smaller
Good:
Slightly less friction
Longer chain/sprocket life
less chain tension
Slightly lighter
More log-jumping clearance
More chainstay clearance
Bad:
Slightly heavier
Chainstay clearance may be a problem on some frames
Rapid chain/sprocket wear
Greater chain tension (increased liklihood of the axle slipping in the frame)
These differences are mostly pretty minor. Most riders will be best served by a chainring somehere in the 30s for technical off-road use, 40s for road or bike path use, low 50s for track use.
Since 42 tooth rings are very commonly available on road cranksets, this size is particularly popular for conversions.
If you use a flip/flop hub, running smaller sizes gives you a bigger gearing difference for each tooth difference on the flip flop.