We have a c3 picasso, with a rear facing seat on the passenger side for a 3 year old (with a block to deal with the floor storage problem). Plenty of room for the passenger.
We spent a few weeks in Aus/NZ and bought a couple of car seats there (cheaper than renting them), which was odd -- they were rear facing and belted, but had straps to go over the back of the passenger seat and connect to an anchor point in the boot (trunk).
The middle I never even worried about, the problem I find is that they take so much space in the front/back direction of the car. I.e. if you put a rear facing seat in the front passenger seat, no adult can sit behind it, and if you put a rear facing seat behind the front passenger seat, then no adult can sit in the front passenger seat.
So for the first child, a compact was no problem. The read-facing seat takes the non-driver side of the car - front or back doesn't matter). One parent drives and the other sits behind the driver.
With TWO rear facing seats however, we found that not even a medium size car (A4, 9-3, C-class, 3-series...) would fit them in a way that also fits two adults - at least not in a way that works for long trips.