Dry chem is reasonably safe, its mostly bicarb soda. If its a fuel related fire, no amount of dry chem you can realistically carry will put it out and you've also got the problem of what ever was hot enough to ignite the fuel is still hot enough to ignite the fuel (chem takes away the o2, breaking the chain reaction, hot leaky fuel ignites pretty much straight away when you stop). And Pigsti's concerns are kinda correct it is hard to clean off, but better than the alternatives unless your going to carry a big foam extinguisher
Carrying a small extinguisher is great but its crucial to get to what ever is burning quickly and put it out, once it's going you'll need the fire brigade. There's also the danger of opening a bonnet with a fire underneath flaring up, So be careful 30 seconds to put on a cotton/wool jumper and some gloves (leather, those plasticy mechanic gloves burn really nicely) if you got em is better than the rest of your life with burn scars everywhere.(most of the fires air will be coming from under the car either way).
Also about once a year take out your extinguisher to check its still charged, then tap it on the ground a few times and check it again. the chem cakes up almost solid after time and becomes useless.
I usually carry a little 4.5kg dry chem, but i got it surplus, smaller one would take less space.