Windows 10 Resize monitors in Multiple Displays

[Aashishkebab]

Windows gets the display size here based on the resolution.
3 is a 4k monitor and 1 is a QHD ultrawide. However, 1 is actually way larger than 3 (and way wider).

Inside Windows:
Actual setup:

I need to make monitor 3 a lot smaller (and 4 bigger).

 

[Neemobeer]

Physical size can not be changed since the screen sizes are as you stated based on supported resolution. At most you can adjust the scaling.