The Atmel ATTiny2313 springs to mind. It is supported by several third party arduino cores and has a Hardware UART module along with an 8MHz internal oscillator.
It is larger than the ATTiny84 or ATTiny85 in that it has 20 pins, but it does have a hardware UART which the others don't.
Datasheet: http://www.atmel.com/images/doc2543.pdf
You could use an ATTiny85, but you would be limited by the fact that it requires a software based UART. This isn't a major issue as you are using Half duplex. The Attiny85 is an 8pin uC so is very compact. There are 5 digital pins which should be enough (TX, RX, Direction, and your two 5v signals)
My attiny core does have a software serial port built in which runs on an interrupt so fairly well represents the hardware UART with the limitation that it is half duplex, but that shouldn't cause you an issue. The core can be found here: GitHub - TCWORLD/ATTinyCore: ATTiny Core for Arduino 1.0+