Be aware that, depending on the design of your car dash, there may be little room and heat build-up could be an issue.
However, if you plan to install in the boot (trunk), the size is probably not so important. If you are operating on a tight budget, you may even think about using a cheaper desktop model, albeit with a different power supply. Follow the link for data on 12v DC:DC power supplies.
Other popular mounting locations are under one of the front seats, and under the dash
Many people opt for the Voom range of auto PC cases. I personally use the one shown in the photograph to the right. It is strong, compact and has a good range of additional input/output sockets built into the end plate.
Cases for automotive use are normally designed with either chunky fins or extensive ventilation slots to help dissipate heat, since there is less space inside the case making it more susceptible to heat build up.
The number of cases from which to make your choice is much smaller than for desktop installations. This is a much more specialised area. The list below shows some of the manufacturers who make custom computer cases specifically designed for Mini-ITX boards.