[Mick Sutherland]

Here are the measurements for the seats, assuming that mine are correct.
From smallest to largest:
-125 x 302mm, board 18mm thick, thickness including board 70mm
-300 x 317mm, board 18mm thick, thickness including board 70mm
-357 x 357mm, board 18mm thick, thickness including board 70mm
-355 x 645mm, board 18mm thick, thickness including board 70mm
I hope this helps