For gearing, use a ratio as close as possible to your favorite combo on your roadbike. For me, I found that I lived on my 42-17 but wished that I had a 42-16. My rear wheel came with a 15t fixed cog and a 16t freewheel. So I got a 40 tooth ring for up front and that's been working pretty good.
You can calculate gear ratios here:
http://sheldonbrown.com/gears/