Prepare
Fit pages to A5, A4, A3, Letter, Legal, Tabloid or a custom size. Proportions are kept unless you ask to stretch; orientation follows the page or your choice.
Pick a named size or type width and height in millimetres.
Fit keeps proportions and centres the page; stretch fills the sheet exactly.
Auto keeps each page's orientation; force portrait or landscape when you need to.
The defaults come from your file. Change a setting and the proof updates before you let go of the mouse.