How do i make it change ALL pages sizes to the same thing?
Libreoffice Writer - Format page is just changing CURRENT page’s size.
If that’s the case, you must use multiple page styles. Change the page size for all page styles in use ( check in in styles manager selecting Applied Styles at bottom of the pane and selcting page styles which is the 4th icon beneath word Styles).
If it changes only the current page, your document probably qas converted from .docx. Since Word has no notion of page styles, the conversion creates a real mess. You can check the existing page styles in the style sidepane. If you see numerous Converted123, you were victim of conversion.
First save as .odt. After that, attempt to restructure your document, removing excess page breaks. Restyle with adequate page styles. But be aware that recovering from conversion hell is very tedious.