I actually carry 15lb dry chemical. It is ABC rated which refers to the three classes of fire it will put out. B is the most important for vehicles, which is liquids but having all three is a plus due to electrical or interior materials.
Then you will most likely want one smaller than mine. I would check off road type stores. I recommend the largest one you are willing to carry that is dry chemical ABC rated.
I have a one pound Halon bottle under my front seat and a five pound Halon on my rear door on my Explorer.
All other cars have a five pound Halon. Plus a 25 pound Halon in every garage.