I'm not sure that I understand the query. Why have you put this in the CMS section? More importantly, PHP runs server-side so has no inherent knowledge of the screen or window dimensions for any client. There are workarounds, of sorts, but PHP cannot dynamically adjust the page breaks for changes to the window dimensions. The best you can expect is to paginate for a fixed page size, which may or may not correspond to the actual display.
Also, the accessibility features built into the browsers allow the users to vary the font sizes anyway. So you cannot even rely on the default sizes of the text corresponding to the actual display.
Basically, web sites are not designed to do what you ask.