Human life is woth more than any car and small extinguishers are useless for fighting car fires, an AFFF extinguisher or large Dry Powder will work well but both are usually too large or outside the ability of the average user.
For motor sport use the MSA requires that all cars carry a fire extinguisher. If a hand-held unit is used the specified minimum size is 1.75 litres; only AFFF is permitted for hand-helds. Small, but considered acceptable by British motor sport's governing body!
Dry powder extinguisher will knock down a pretty big fire; for car fires which it's practicable for the driver to fight a 1kg bottle is probably adequate, although 2kg is better - if it needs more than that, just get as far away as you can.
One of the problems with car fires is that they are likely to be caused by an electrical fault; there's always the possibility that the circuit causing the problem will be permanently live, in which case switching the 'ignition' off won't remove the source of ignition; if you can't remove the source of ignition,it's likely to re-ignite.(Not such a problem in motor sport as MSA regs require an externally-operable electrical cut-off switch which isolates all circuits except any required for fire extinguisher operation.) For this reason dry powder is preferable to AFFF; AFFF is, in any case, a pretty poor extinguishant - good for cooling things down & blanketing petrol spills, etc.
A point worth remembering with dry powder extinguishers: in time the powder will compact. Take it out of its mountings once a week or so & turn it end over end; you should hear or feel the powder moving - it it doesn't, tap it gently to break up the lumps!
Just a few observations from a motor sport marshal who has actually put car fires out!