If you look closely at the ABC ones it will tell you how big of a fire it will put out of each type. The one I carry says "1A:10-B:C" and is 2 1/2 pounds. A-combustible material (wood etc);B-liquids on fire (like gas); C-electrical fires; D-flammable metals like magnesium etc (Probably don't need a D rated.
The 1 in front of the "A" means it will put out 1 cubic foot of material, in front of the B it means it will put out 10 square feet of burning liquid the C just means it works
Keep in mind a typical fire will at least double in size every minute. So if you have a 1A that is going to take you a minute to grab you might as well stay and watch it burn
I've only needed "D" twice and didn't have it either time. It was when some newbies left the park brake on my snowmobiles and drove off. The rotors are made of magnesium so once they got hot enough it burned them both to the ground. I got a new one one year and the second time was a year later on my other sled so I got a second new one but was dropped like a stone from my insurance company
My shop is a 10 pounder that is 4A:60B:C. It was given me by a friend who works for Treasure Fire Extinguishers. He told me anything less than 10 pounds on a car fire is just going to piss you off. I've only done one car fire and it was just a couple of wire and my small one worked great.
Use this knowledge for what it is worth. I'm am not an expert but have graduated from the "fire school chapter" in my CERT class. One thing for sure is to have it secured in the vehicle properly since they always seem to be attached around a human being (roll bar, in between seats, etc)