You can make it as small as you want. If you don't need USB interface for the final project, than its hard to get much smaller than a Promini.
For example, here is a Promini installed on a perfboard on a wirewra socket, connected to some other parts.
When I ordered 20 prominis back in September, I didn't know as much about them, I am down to my last 2.
I think next project I might go with standalone ATmega chips, two 22pf Caps, 16 MHz crystal, and 10K pullup resistor and wire that up board like this instead (or maybe make a PCB) - all available really inexpensively from dipmicro.com. Maybe a header connector for programming. And decoupling caps.
If you don't need USB, you might also consider a bare PCB from mouser for the older serial card, fill in the thru hole parts you need (don't need true RS232? leave off the connector, transistor & other parts for it!).http://www.mouser.com/ProductDetail/Arduino/A000006/?qs=tbM5a8K56PXxkRNA99BjOg%3d%3d
Or for not more $ & one-stop shopping an ardweeny from solarbotics, RBBB from moderndevice,