Rhots:
There are some concepts which may help to understand what is happening:
What is printed is the screen image and not the model. The display nor your printer under stand model size. They work strictly in pixes / dots per inch;
For the fit to page print what you see for page size is not the model but the screen size SU has calculated based on its knowledge of the model ize and the pixels size of your screen settings;
SU then uses that information and the printer settings to determine the scaling it must a apply to print the screen to your specified paper size and thus the scaling you see in the left scaling box.
BTW I would suggest you toogle the use screen extents several times until the the page size report is consistent. ( I think there is a bug here because this and the zoom extents in the SU window give different results. )
In addition related to your comment about dimensions disappearing check what you have set in the expert dimension dialog box. There is an option of not printing when they get too small. πΏ
I would suggest you make a simple rectangle model with an aspect ratio consistent with you screen (2d) and then make its size very close to the paper size and check out what is happening including change of print format.
Just some thoughts based on what I think I know π