The ARM version of uLisp now supports the Adafruit Gemma M0 board, which must take the prize as the smallest board capable of running uLisp:
It includes an on-board DotStar RGB LED that you can drive from uLisp, and has three I/O pins which can be used as digital inputs/outputs, analogue inputs/outputs, an I2C interface, or a serial interface. It also includes a socket to allow it to be run from a Lipo cell.
For more information see: Adafruit M0 boards.