Compact carseats for medium car?

Baby #3 is due in 7ish weeks time for us and we are looking at buying a new car. It is likely to be a medium sized car and we will not be able to get a bigger car for some time.
At the moment we have two SnS carseats for our 2 & 3 1/2 yo. I had planned on just getting a compact booster deat for DD and giving baby her reversable carseat. But it look as though we may need to get 3 new compact weats as the two SnS seats are quite wide.
So which seats are more compact, including booster seats and baby carseats. Please help as we have no way to get to a store to check carseats out and test if they will fit or not at the moment!

Kell, I think the ones from Target and Kmart are the narrowest from memory...do you want me to take my tape measure down and measure them?

I don't know if it's possible, but maybe you can measure the carseats you have now, and do a little searching online for different brands, and check what their width measurements are? That way you can see if the carseats/booster seats are bigger/smaller than yours, and make somewhat of a judgement from that?

That way when you actually get a chance to go to the shops, you already have an idea of what brands or particular seats are suitable/not suitable without having to leave home :)

Hope I helped! :)