I have the Belkin TuneFM, and it's pretty small because, unlike some of the other ones, it displays the broadcast frequency on the iPod's screen, rather than requiring its own display. I don't know if it's the smallest, but it may be. But be aware that Belkin has other solutions out there, too, that have the transmitter integragrated into a bracket and charger. Those probably do take up a little more room, but the unit I have is just a small transmitter with an optional charger that hooks into the cig. lighter. If you don't use the charger, it runs off of the iPod's battery.
But to be fair, I don't think any of the available chargers are much bigger than the Tune FM. If you're worried about space in your car, I'd guess you're more worried about the mounting solutions that come with some of the transmitters. Just pay close attention to how the unit is mounted, or stay away from transmitters with integrated mounting solutions, and you should be fine.