Ed, I was in the same boat not that long ago. I was trying for older sidewalks in industrial areas, so a little more narrow and small. 3/8", while most correct with my measurements, it struck my eye as too large, 3/16" too small. I went with 1/4" square, .040" square curbs, Model Master aircraft grey and India Ink for color:


I'd agree with what you are doing in an urban area that multiple sizes would be best, looking forward to the results!
Lucas