3.4 meters in length, 1.48 meters width, 2 meters height, and a cap of 660cc on engine displacement. If the car falls under those restrictions, it is categorized as a Kei-car in Japan, and is liable to receive lower taxes and a much lower insurance bracket.
Devised as a solution for Japan's crippling traffic problems in major cities, the Kei-car's cub-sized presence makes them popular as a daily driver. They tend to be efficient, take less space to park, and are fun to zoom around in if they have even a slightly potent engine (like the Autozam AZ1, Suzuki Cappucino, etc). Don't take them out on the highway, though, it happens to be a seriously scary experience.
The Rush is a mini-SUV, the Kei version of regular SUVs. The last generation Daihatsu Terios Kid, was a kei-version of the regular Terios.
Here's a thought: why don't we have more kei-cars playing our streets? It would save a lot of the congestion problems, and they'd be cheap and inexpensive to maintain, too. We have a feeling it's the bulging waistlines of the car-buying crowd that limits their interest in kei-cars.