I find my 32" 4K scales poorly in macOS, basically I can have everything tiny or everything massive and the scaling options make no sense there's only 720p, 1080p, 1600x900 or 4K.... there is no option between 1920x1080 and 3840x2160.

In Ubuntu it is more flexible but I think the sweet spot is going to be a 6K for scaling the UI at 3K but those might be a year or two away from existing and affordable. At below 30" 5K would be fine, I have a 25" 2.5K monitor that is very comfortable to use.