Thanks for all the responses. As Ratslinger first noted, the solution is found in Format ->Page Properties… ->Page. HerCorns said Format → Pages… ->Page; is that the Windows or Linux version? As noted in the OP, I’m in MacOS (10.12, the 2016 version). Why would such a simple thing be done differently in different platform versions of the application? Seems needlessly confusing.
As Mike Kaganski notes, indeed, the page size setting is document-wide; change it for any page and it changes the whole document. Which is fine for my purposes.
Anyway, as this is my first adventure with LO, what I found interesting was the difference in style. I’ve been in the Mac world exclusively for going on 30 years, but had heard that in Windows (and I’d guess in Linux, which generally follows Windows ways of doing things) procedures tend to be application-specific (and sometimes widely varying) where in Macintosh, where applications are more thoroughly integrated into the ecosystem, they are done globally in the OS. So this is an example.
Puzzled: ajlittoz wrote: “…change specific page styles which you access with F11, fourth icon in the toolbar.” Is that the fourth icon from the left in the toolbar across the top of the window? In the toolbar I see (default, unmodified) there is nothing labeled “F11” (the fourth icon is Export), nor is there anything with that name in the list under View ->Toolbars ->Customize…. If the reference is to the F11 key in the top row of the keyboard (MacBook Pro, early 2013), pressing that either (a) raises audio volume, or (b) (with fn key) clears the Desktop by moving all windows to the edge of the screen. No connection to the LO application.
However, while researching this, I ran into another problem: I inadvertently tapped on the icon for Zoom & Pan, which turned the cursor into a magnifying glass (over the document) – and then could find no way to turn it OFF. Clicking again on the tool icon didn’t do it, nor did double-clicking. I searched in the Draw manual for “Zoom & Pan”, found nothing; searched “Zoom” and found a bunch of mentions, but nothing relevant. The latest Draw manual is for v.4.3; maybe this tool is newer? Anyway, how can it be turned off? I finally figured out that if I closed the document and reopened it, the tool reverted to off; but I can’t believe this is the official method.