MS Word page size suddenly changed and no way to get it back

Hi,
the page I was working on suddenly changed its size to smaller vertical and I can't find a way to retrieve original size which was A4. This new size has become the default size and all the files that I open are modified and text is lift up for about 3-4cm out of the page. I searched all menus without success. For now, on the attached image, the A4 size is 25.5cm high instead of 29.7cm.
thanks for your help.
(Windows10 - Office365)

Paper Size = A4
Total Size = 21.0 x 29.7 cm
Margins = 2.54cm, 2.54cm, 2.54cm, 2.54cm
Document Area = 15.92 x 24.62 cm

I think I duplicated what you are seeing and what setting caused it.

Hi MisterEd,
Thanks ! That's it !
...althought I'm sure that I didn't access to this option window. This happened as I was tweaking the title. Maybe I did a wrong keys combination which was a shortcut for this?

Have a nice day

Philippe