I designed a USB to RS-232 adapter this evening. There's no particular reason I did. It was just sort of "because I could." It was actually sort of tricky to route, since the RS-232 level converter chip wasn't conveniently laid out - like with RS-232 signals on one side and TTL/CMOS on the other.
My first cut wound up being a 2.08 x 1.25 inch board. It seemed like it could be smaller than that. Most of the space was taken up by the through-hole DB9 connector, so I swapped that out for an edge-mounted DB9. The only thing attaching the connector to the board will be the soldered pins, so it's quite a bit weaker, but in principle such a thing would be mounted in a plastic case designed to relieve any strain. I got the board down to 1.8 square inches. I used an SOIC RS-232 level converter instead of an SSOP one, so there probably are still gains to be made. In principle, it's conceivable that one could attempt to place two SSOP-28 packages on opposite sides of a much smaller board. That's probably how they achieve a USB-to-RS-232 adapter that fits entirely inside of a DB9 shell. I don't feel a need to go quite that far. Heck, I don't really have a particular use for this anyway. I'm not sure we have an RS-232 connected peripheral left in the house.
I'm not really sure why I set down the road to make one of these. But I've got 3 on order, and I can sell them for $15 each (case and micro USB cable not included), given the price of the boards and BOM.