By full width do you mean full width of the browser? I think you would just put a very large maximum width in Style Properties > Page setup > page width.
But if you mean no sidebar, then by default a page node is full width because there is no sidebar (unless you add one by adding some widgets ).
But if you mean something like the forum list page, you just need to make sure there are no widgets assigned and the there will then be no sidebar.
Let's pretend this screenshot represents a page (I understand it is not a page, but again let's pretend it is). I want the width of this page (not all pages, only this one) to be 100%.
What is in red is the current width I'm looking to change from X width to 100% / what's should fill the green rectangle:
Let's pretend this screenshot represents a page (I understand it is not a page, but again let's pretend it is). I want the width of this page (not all pages, only this one) to be 100%.
This is something I do but I do find it better to make an extra style and choose that in the node setting to override the default style. If it's a child style it is easy just one setting, no maintenance required.
Yes you just disable user selection, it won't appear in the style chooser. I've also used for for noindexing specifically pages by editing PAGE_CONTAINER for a no index child stylestyle.