I've been learning a lot over in the classic & vintage section, but am totally new to the fixed gear world. I've got a couple of early 70's Schwinn fillet-brazed Super Sport frames, that I thought about converting to 6 or 7 speed commuters.
But now I'm thinking about saving even more weight by turning at least one of them into a fixed-gear rider. So, I guess my first (of probably many) question might be what size front ring and what size rear ring would a 210 lb. rider want for a mostly flat terrain? He is young (my son-in-law) and probably considered pretty aggressive, speed-wise.
Gearing is very personal. I suggest starting a new rider out on 48/18. Larger gears are more difficult to accelerate from a dead-stop and are also more difficult to decelerate. You won't achieve blazing fast speeds, but that's not the point. Control of the bicycle is more important than top speed. Err on the side of caution.
I would start at 48/19 and learn to spin, control the bike and it will give you a lot of skid patches too. When you loose weight and are confortable spinning then I would get to 48/17, I find this ratio the best, not too low nor high.