Just been out, even with a measuring tape it's a bit subjective but here goes.
Total width of seat at widest part - 1,180mm
From seatbelt to outermost part of seat - 460mm on each side
Middle seat between the seatbelt fasteners - 260mm.
In reality the person in the middle seat would have a bit more on each side - if you said 25mm on each side that would give 310mm. And the 2 on the outer seats wouldn't quite be up against the door so would have say 400mm each. Definitely only for a smaller person in the middle.