They sell a "one shot" type for a reasonable price. But small ones can only do small jobs. A small home-type extinguisher would fit on the back of a large crash-bar, under a luggage rack, or the back of a sissybar. A little bigger one would fit in a saddle bag if you have em'. It becomes a matter of "How much other stuff do I not want to carry?"
A first-aid kit is more commonly needed than an extinguisher. What about water in case someone is dehydrated? Road flare? When good people see something awful, they naturally want to help. But there's only so much you can do, and only so much a motorcycle can carry.
I think that you can still be useful without carrying safety equiptment. Making sure the people with the car fire are safe and away from the fire is great. Cars can be replaced. Calling 911 for them if they need it, or the road-side assistance number if their phone got left in the car. If the car is ON the road, it might be helpful to go back down the road a bit and put down a road flare, or wave off traffic from the lane they're in. Heck, if they are alone and freaked out, just being there with them until help comes could be helpful.
