OK, I need some help.

Basically I use the Amazon Kindle App on my computer. I bought a 24" 4k (higher ppi) monitor I use portrait mode (I turn the monitor vertically), to read my kindle books from.

However due to my field of vision (and the monitor isn't curved), and the distance of the monitor (which can't be changed). I need too cut part of the bottom and top of the kindle app off.

Now in window mode I can do this with the simple dragging of the top and bottom part of the app. But I like the Kindle Apps full screen mode where it hides the menus and stuff. But you can't cut off the top and bottom using full screen mode.

So is it possible to run my monitor's resolution at it's native resolution. But trick the program to think my monitor's running a different lower resolution. So when I use the full screen mode, it actually only takes up part of the screen.

Basically I want full screen mode ran in windowed mode.

Also note I don't want to run the program in windowed mode without borders (using the borderless gaming app). Because the Kindle program's full screen mode hides other things than just the borders.

Just to clarify, I want to change the aspect ratio of a program, so when in full screen it only uses it's part of the screen.