EDIT: I figured it out. I needed to close Discord before adjusting video settings. Read the stickied posts, kids.
I changed my display to Windowed mode last night. When I tried to change it back to Fullscreen mode this morning, I get the following error: “World of Warcraft was unable to start up 3D acceleration.”
I am similarly unable to change the Render Scale/Window Size, as I get the same error (I am able to expand/contract the window on my PC – but cannot do so via the in game menu).