How to enlarge GUI text & icons on my 4K screen?

[nop]

Avidemux is really hard to use on my 4K screen, & none of the Ubuntu settings or tweak-tools will enlarge the text or icons. Any tips?

[eumagga0x2a]

Which Ubuntu release and which Avidemux version do you refer to? If you've compiled Avidemux with Qt 5.14.x or newer, you can adjust font DPI separately by setting env var QT_FONT_DPI:

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QT_FONT_DPI=120 your_command_to_launch_avidemux
(use the value which matches best instead of 120). All versions should respond to setting QT_SCALE_FACTOR to 2 and enlarge also all elements of the GUI accordingly (don't use non-integer values!):

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QT_SCALE_FACTOR=2 your_command_to_launch_avidemux
The official appImages (use those with GLIBC in file name from the appImage4Buster directory for Ubuntu 20.04 or newer, else resort to legacy ones from the appImage4 directory of the nightly server) won't support the former but should support the latter.

edit: If you have more than one display, please follow https://wiki.archlinux.org/index.php/HiDPI#Qt_5