Small (max 5x5 mm) connector, panel mount, for asyncronous serial interface?
The CP2102 USB to UART Bridge has an unbalanced 3.3V interface so you should be connecting ground in addition to the TX and RX lines. Using a shielded cable probably isn't a bad idea although at fairly standard low baud rates such as 9600, 57600 etc noise immunity is fairly good so you may not have to worry a lot unless the application is critical or you're in an electrically noisy environment. For test purposes I've run 3.3V TTL serial lines over a few meters of unshielded cable without problems.

A connector I've used in the past that is quite robust and offers panel mount in a small form factor is the Binder 709/719 series. The 3-way panel mount part number can be found at Element 14: Binder 09 9750 30 03. They are a little larger than your specifications and not exactly dirt cheap but are very easy to solder to, here's an indicative photo that shows the 4-pin variety:



Your idea of using a 2.5mm audio jack is probably also worth considering although you need to be aware that while inserting the plug some pins can be temporarily shorted. The RX line being shorted to ground or TX won't matter but you should take care with the TX line at each end. Without specific details of the hardware at each end it's hard to know for sure but a 3k3 resistor in series with the TX line at each end will limit the current to 1mA which should be safe for most devices I can think of.