Small "FTDI"-like chip
The only way you are going to really save space is to just use a microcontroller already on your board. This can easily be done by using a microcontroller, which I am sure you have at least a few of, and make sure one was USB. Have it handle the USB, no need for an extra chip.

If you are wanting what FTDI does, as well as FTDI does it, with the same thing but a smaller chip, you are going to be disappointed. The only other way to get around this is remove the need by not using a TTL connection.