The Toyota Yaris is the smallest and the next size up is the newer Nissan Versa that I have not tested but will to bet that it will work in Fail-Safe as well. The problem is the smaller the EPS, the smaller the plastic driven gear and may not take the load of a bigger car. Worst case the gear breaks, you are back to manual steering.
The best bet is the Prius and Corolla as they will be much easy to find.
Waid
