Follow these steps in order to change your fonts’ size in the Finder or Mail sidebar:

  1. Launch System Preferences by clicking the System Preferences icon in the Dock, selecting the System Preferences item from the Apple menu, or opening Launchpad and selecting the System Preferences icon.
  2. Select the General preference pane from the System Preferences window.
  3. Use the drop-down menu next to the “Sidebar icon size” item to set the size to Small, Medium, or Large. This drop-down menu controls the icon and font size for both the sidebar in Mail and the sidebar in a Finder window. The default size is Medium.

When you’ve made your selection, just close System Preferences.

Posted in Lion, Macintosh Tips & Help