There was a question a few days ago, and one answer referred to this page, giving advantages/disadvantages of using larger or smaller gears for the same ratio:
http://www.sheldonbrown.com/fixed.html#bigsmall

Another consideration for fixed gear is the number of skid patches. Say if you have a 48:16 gearing, every time your cranks go around once, the wheel goes around exactly 3 times. So if you lock your leg up at the same place in the stroke each time, then the wheel will always be at more or less the same spot, so you wear that one spot more. Of course, there's no law that says you have to go skidding around. On a coaster bike, that makes no difference, and my cruiser has 2:1 gearing with no ill effects.