Don't get a small extinguisher. It's a bad feeling when you're trying to put out a fire and it runs out.
A large 10ABC rated bottle or don't bother.
Hmmm...
It's true that ONLY a small isn't ideal...
**But I have personally stopped two car fires in early stages with two different VERY small extinguishers.
-In fact, in one of those two scenarios, the fact that it was small was the only reason I could aim it at the source.
With a big one, I could not have created an angle to access the source.
It's all about access angles.
I agree with the basic thought & carry multiple extinguishers...small...medium...large, but don't discount the power of a small.
In my experience (and I've dealt with no less than 5 car fires--only one of which was unstoppable), if you can get at the source? -It's literally OUT with a blast of LESS than one second. 3 of the 4 that were successfully stopped were almost instant, and in only one case did I ever completely empty the extinguisher (it was a small one that emptied, but it stopped the fire).
My largest extinguisher has a hose attached for better aiming...but in the above example, it wouldn't have helped, as I was literally reaching up and under a fender well to get at an oil fire (hood couldn't be opened).
Here's a screen grab from a video I shot of the fire that couldn't be stopped, but we WERE able to detached the towed vehicle from the hitch before it could also engulf the RV that was towing it: