Right click to adjust audio gives me the box below. The only resolution that shows the complete box is a 4:3 aspect and still doesn't resize properly. Box below looks the same at any 16:9 resolution.
What is the OS, and the vertical resolution of your screen? My laptop and desktop all have 1080 or better, no problem like this. I know the specs have listed 800 as the minimum vertical requirement.
Windows 10 - all updates to date.
Resolution doesn't matter, whether it's 4k or 1080. The wasted space in the UI is bad enough, but this is just another stupid annoyance that should exist. Beta behaves the same way. As it's not widespread, I'm confident it'll never be fixed.
Tested on Beta 220.127.116.11. Win 10 2004.
Right click media file in Project Panel. Modify -> Audio Channels. Displays full window on 1920x1080 and 3840x2160 displays. However, the box cannot be reduced, width or height, beyond a point (probably the original default). It could be the height of 800? The window can be enlarged in either direction independently. Once changed (and exit with "OK"), next use results in changed window size.
I don't see a way to get what you are seeing with scaling the monitor. Odd.
Curious: looked at 2018.1.2. Same behaviors.
Interesting as you can't modify audio channels from the PROJECT panel at all, this is form the timeline. I included the screenshot so they're wouldn't be any confusion or paid commentors telling me to do stupid things that don't help whatsoever.
Also using any browser with all blockers turned off press the "...More" button shows a blank rounded edge box disallowing correction of typos. I love Adobe, nothing but frustration and excuses.
Don't know what "paid commentators" you're expecting.
What is your screen's pixel resolution? Minimum is 1200x800, some laptops fall below that still.