I think I was the one who raised the issue of a car seat fitting properly. I was referring to the seat pitch (the distance between rows) that was an issue once for us, so it's the length of the car seat that was the problem. It's of course wise to check the width, but as others have stated, that should not be an issue in a row without fixed armrests.

To illustrate, in our case, the couple inches you see between the car seat and the seat in front in the photo below just wasn't there. Our seat just wouldn't fit that way. It did fit with forward facing due to the curvature and the angle of the seatback.