Finder and Mail Sidebar Size
If you find the font and icon size in the Mail or Finder sidebars a little too large or too small, you may be able to change it to one that’s a better fit for you in the System Preferences, although your options are limited. Apple consolidated the size controls for the Mail and Finder sidebars in OS X Lion in a single location in the System Preferences and later added additional applications with sidebars to the mix. The centralized location makes it easier to change the size, but you’re limited to a single choice for multiple applications. The size that works best for one application may not necessarily be the best size for another. Nonetheless, you can make only a single size choice for all the affected applications.
iTunes or Music Sidebar
With the release of OS X Yosemite, Apple added the sidebar for iTunes (which became the Music application in macOS Catalina) to the same system preference that controls the Mail and Finder sidebars.
Photos, Disk Utility, and News
With the advent of OS X El Capitan, the Photos sidebar and Disk Utility sidebar were added to the same system preference for controlling the size of the icons and fonts used in the sidebar. The News app, which was introduced in macOS Mojave, also has a sidebar controlled by the same preference.
Changing the Sidebar’s Font and Icon Size
To see the effect the sidebar size change has, open a Finder window and the Mail app (or any of the other relevant apps) to observe the size change on the fly. Both the sidebar icons and the accompanying text are affected. If you find the global control of multiple apps’ sidebar size a problem or if you think it’s a great idea and should be extended to more Apple apps, you can let Apple know using the Apple Product Feedback form. Select macOS in the macOS Apps section as the feedback form to use.
