There aren't very many places in the C3 interior where a fire extinguisher can be located which is both easily accessible and not in the way. I mounted mine sideways, on the floor just ahead of the passenger seat.
As for what size, you want the biggest fire bottle you can mount without it being in the way.
An extinguisher in a vehicle should be ABC-rated. 1-A:10B:C is the size which easily fits in that spot. I have a 2-A:40B:C mounted there.
As for dry chemical vs. "clean" extinguishing agents such as "Halotron"...extinguishers using either are effective but dry chemical bottles, while less expensive, leave a mess of white powder. For a given size of fire extinguisher, dry chemical will put out more fire. For example, a 6-lb dry chemical bottle is rated 3-A:40B:C where as a 6.5-lb Halotron bottle is only rated 5BC.
So, if you really want to put out a vehicle fire, you should have a dry chemical fire extinguisher.