If you want to buy a second smaller car that's fine, but just don't do it because you think it'll help the environment, because it'll actually have the opposite effect. Producing a car takes a huge amount of energy and creates a huge amount of carbon pollution and other waste. The amount of gas (and thus pollution) you'd save by using a new small car will never come close to making up for the pollution that was created in the production of that car.

The most environmentally helpful choice is for people to keep using their current car for as long as they can, no matter how much gas it guzzles or oil it leaks.

With that said, life is of course much more complicated than that, so it shouldn't be the only factor you consider in deciding whether or not to buy a new car.

EDIT

To the couple of people who downvoted this, I'm not just making stuff up, this is information I heard from scientific sources. Here's citations:

https://www.scientificamerican.com/article/when-used-cars-are-more-ecofriendly/