I am aware you can go into pref and change the font size for the message list. That of course only edits the fonts in the email. How to you adjust the fonts in the two columns on the left of the window?
Hmmm....
"Message List Font" is greyed out in Mail Preferences. There is nothing about the Mailboxes font there at all. I tried to search the Help file, but came up empty.
If you switch to Classic view you can change fonts in the message list. To change the font in the folder window, go to System Preferences > General > Sidebar icon size > Large. I find large to be too big. Long for the old days when you select the size of font.