Hello,
No, the width and length are fixed (so 135x55cm). Only the height is adjustable.
When they say that you can fit a tray up to 180cm long, it's so that there isn't too much space under the tray, to avoid it becoming brittle when you put weight on it (after all, it all depends on the material and thickness of your tray).
I hope I've answered your question correctly, have a nice day! :-)