Today, I took the car out for lunch (as in, I ate lunch and filled it up with gas) and parked it back on the other side of the garage as I switched back to the Ford Five Hundred after my extended test drive.
I conclude that I can comfortably drive a Ford Fusion. I have no idea when I'll be replacing the Five Hundred or exactly how big Katie and Julie will be then -- and that will be an important consideration when I get the next sedan, because I remember having spent too much time in a too-small back seat. (The back seat of the Fusion is decently sized, so we'll likely be ok, but I want to be sure about that.)
Overall, a successful test.