I have ridden 42,45,48,50 x 16 and have mostly settled on a 48*16.

If you like to sprint, 46*16 won't be a big enough gear. Drafting busses and other cars sucks in a gear too low. I like 48*16 because I can get up to low 30's easily and can maintain a 21-22mph average.

One other thing I consider, where I live, downtown traffic moves at between 25-30mph when there is not too much traffic. I like to ride *IN* traffic and take the lane. That means I have to be able to get to 30-35mph and keep it there. 46*16 won't work. 48*16 barely does, 50*16 (as another person posted) is better, but makes standing starts a little more work and I don't ride in traffic *that* much.

Good luck with your build. New bikes are fun.