That's indeed strange. Using the same version here, it does work. Wondering how you want to change those settings.
Spinboxes at "the custom" size should be editable, but you cannot edit the existing templates from the list above with the panel.
However default user units are at the top right, would change that first, then the scale factor to 1, then resize the page.
If you used the current stabile.
The absense of the scale factor setting indicates you are using a previous version.
(Re: your
other topic.)
