Scale of palette incorrect for high (or low) DPI displays
For MuseScore 2, we made all elements of the interface respond to the monitor resolution, the physical size of everything on screen is the same across all systems by default (well, except for fonts, we rely on the OS font scaling for that, and provide an override in preferences also).
For MuseScore 3, the code responsibile for scaling the palettes was removed, not really clear why, but it happened as part of an apparently unrelated commit here:
As a result, all 3.x releases so far (both "old" and "new" palettes) have had inconsistent scaling of the palette items, with the size of the palette elements depending on monitor resolution as well as OS & OS settings. macOS typically does the scaling for you, so most Mac users probably see the correct sizes, and users with "normal" monitors on Windows or Linux generally do as well. But users on high DPI monitors on those systems generally see palettes that are too small, and users on TV sets or other low DPI monitors see palettes that are too big.
I plan to reinstate the palette scaling code, so everyone gets the same size again. The correct size being, 100% scale on the palette itself (like on the keysigs palette) means the display of the symbols within the palette is the same size as they actually appear in a score when viewed at 100%.
However, depending on your actual monitor resolution, and any scaling settings made in your OS, it is possible your palettes aren't unusably small, and you might find the return to "correct" sizes to be disconcerting. I find I actually prefer the palettes slightly smaller than life (so I'm happy that a number of them are already set to 80% or less).
In order to reduce user pushback, and provide new flexibility, I also propose including an advanced preference to control palette scaling relative to the correct nominal size. The default will be 100%, so by default, everyone gets a life-sized key signature palette regardless of OS or monitor resolution. But users who prefer smaller palettes (or larger) can turn that setting down (or up).
I have this mostly implemented and will submit a PR soon as a WIP.