The EzCube is by far the smallest FM transmitter we've had our hands on, measuring just 1 inch cubed. Most MP3 players will dwarf the device, with the exception of the MobiBlu Cube, which strongly resembles it.
The overall design is among the most compact for an FM transmitter, but it's also slightly messy, with the two cables and a lack of any console-mounting system. During testing, the whole setup was piled haphazardly into the ashtray of the car--not ideal.
The considerable plus is that the EzCube is a very solid little performer. We're pretty shocked that Eznex managed to cram such a strong transmitter into such a tiny package, actually. The unit pushed out to every open frequency in San Francisco fairly clearly