Display Size - Small is grayed out (Adobe Acrobat Pro)

Yes, I thought the same. I tried changing my screen resolution up & down but the small option was still grayed out. Sometimes these limitations are unnecessary. Just allow users to choose whatever they want on whatever screen they are on, maybe they'll like it (even if it doesn't look that good to the devs!).
It's similar to limitations in other applications. I can think of a really good example right now. My phone's Alarm app wouldn't allow me to put the volume to float values like 4.3 or sth. Instead, it'd just let me increase/decrease by 1 unit, so it had to be an integer. And I never understood the reason behind it. Like... why?! And it was always louder or quieter than what I needed. Why would someone limit the user like that? Handling those float values doesn't add any technical difficulty for the developer. And I'm guessing that this display size issue is very similar to that. Many apps are like this and they always bug me out. I'll never understand these limitations... It's only understandable if it causes serious issues for the app (Maybe for security concerns. I didn't want to set my alarm's volume to 10000 but limiting me to only input integers is extraordinarily stupid), and I don't think this display size could raise any security/technical concerns... Instead the developers "assumed" that the user wouldn't like using the small display size on some unknown screen sizes that is not mentioned anywhere, including the link that you provided...