But changing the width of the section in the middle is not changing. Adapt \textwidth narrows the textwidth but does not allow wider margins.
see next example. So how to adapt the page to for example:
[leftmargin 40mm][middle what is left][rightmargin 60mm] 
this should leave 210-40-60=110mm in the middle, but doesn't.
The textwidth is narrowed but the margins refuse to obey their setting.

yours sincerely
dr. Hans van der Meer

setuplayout: leftmargin=40mm, rightmargin=60mm, textwidth=110mm\crlf
abc\dotfill xyz\crlf