As you can see in the standalone Viewer Appearance settings, there is only a place to set "Minimum window width," but I need to set a Maximum width. Here's why:
I like the "Auto-size" checked because, when "Auto-size" is checked, pictures open at their original size, and zooming an image with the mouse scroll automatically enlarges the borders. I use my left and right arrows to navigate through pictures. In some cases, the pictures have a very large size, and when I open them on my 38-inch widescreen, they fill the screen, so I zoom out a bit. But when I right-arrow-click to navigate to the next picture, it again fills the screen.
Turning off "Auto-size" prevents this, but when this is checked, the borders do not resize together with the picture when zoomed, which I need.
Adding a "Maximum width" setting would solve this since I can set it to not fill the entire 38-inch monitor unless I click full-screen.