Font size on Apple Mail

I have not found any way to change the mailbox-list font or size.

I use TinkerTool to make a few subtle adjustments. The developer scours MacOS system preference files, looking for settings that are present, but not made accessible inside various parts of Applications and System settings, If possible, they make those settings available to you.

Unfortunately, the mailbox-list font size is not on their list, and changing the general (system-wide) default font does not help either.

This sounds like a request for Product feedback:

Product Feedback - Apple

Mail > Preferences > Fonts & Colors ...

you can change WHAT font, what size, Bold or not.

In the versions previous to Monterey, the list spacing was extra large, because of reverse-porting from the iPhone.

what MacOS are you running?