If you get a road crank like the Sugino RD, it usually comes with a 48T chainring. I'd go with a 17 or 18T cog in the back. That'll be acceptable for the flats while not being so high you can't climb hills.
If you get a smaller chainring like a 46T, just go with a 16 or 17T. I use a 42-15 on my San Jose. It came with a 17T cog but after a while I changed to the 15.
A one tooth difference on the rear cog makes a noticeable difference and swapping cogs is cheaper than changing chainrings. I gave the 17T cog to a buddy.