Blurry fonts or very small fonts

[billy surf]

When I connect an external monitor, my laptop folder fonts (only when attaching a file to email, uploading files etc.) are VERY small. To get them the right size, I have to make the laptop the main monitor and that makes the external monitors font blurry AND pointer etc. unusable big...

My settings:

extended desktop
laptop = 3840x2160 (this is the recommended setting)

External = 2560x1080 (recommended)

Any ideas on how to fix?

Link https://www.dropbox.com/s/v0gjfc85vxw4dlo/Untitled.png?dl=0