After making a document and setting its properties using File > Document Properties and in the
Page tab using A3 as format I cannot print this format.
The print is shifted and truncated to some A4 format.
Going to File > Print and then looking into the 'Page Setup" tab, I cannot choose "Paper size" because
it does not lit up.
What am I doing wrong or what did I forget to set?

Hi.

I never had any luck getting Inkscape to print correctly. What I have done is to save to PDF and print the PDF file.