How to increase the font size throughout my entire 200 page document?

Is there a way to apply a font change to an entire document, as in
“decrease/increase font size by two points, or four points,” etc? I want to
avoid repeatedly selecting areas of text and then selecting font sizes from
the toolbar.

Thanks,
mango

Open HELP and select FONTS - you will find guidance there.

Open a text document, choose Tools - Options - LibreOffice Writer - Basic Fonts (Western)
Set the font/size you want in the appropriate line; tick ‘Current document only’

You can change the font or size for main body and headings this way.

The example below changes the font size to 15 throughout the document althought there may be some elements that need manual intervention.

as word of caution if you use multiple fonts such as symbol, webdings, all the wingdings etc. i have been unable to change font size without changing the foregoing to the basic font type of the document. furthermore if you select a page, format page is a misnomer as it actually formats the section. hope this helps. ken

Alternatively, use Styles. If all your text is in the Default style, for example, edit that style (hit F11 or the ‘Styles and Formatting’ button) to change the font size and it will be applied to everything that uses that style. It is easy to use different styles for headings, quotes or whatever you like.