I hesitated to jump in on this one since I don't have much direct experience with the vehicles you mentioned. But *very generally* speaking, if all you need to do is have 2 adults in the front, and then one RF carseat plus either another adult or a FF carseat or booster in the back, most cars should be able to accommodate you as long as the adults are not too tall.
We had a Honda Civic for many years and all 4 of us fit fine, even with a RF Radian (one of the larger convertible carseats) on the passenger side, though again the front passenger was only about 5'6" and she had the bare minimum in terms of legroom.
I will say that, based just on my subjective impressions, the Focus does appear to be a little smaller in the backseat than its competitors like the Civic. Spec sheets for the current models list 33.2" of rear legroom for the Focus vs. 36.2" for the Civic, and overall interior volume of 103 cu.ft. for the Focus vs. 107 for the Civic. Sometimes these measurements don't really translate into reality, though, especially when it comes to carseats.
Going up to a midsize car (Fusion, Accord, Camry, etc.) will definitely give you a little more room to work with, but of course that comes at a cost. Whatever you pick, check out the IIHS safety ratings as there are some major differences among vehicles, even in the same class and price levels.
