For Xcode 12: (beta 3)
For the code editing windows, use the new
Editor -> Font Size -> Increase
or
Editor -> Font Size -> Decrease
menu items. This globally increases or decreases the font sizes for all editing windows. There is also an
Editor -> Font Size -> Reset
option. These also respond to the ?+ or ?- keyboard shortcuts.
By default, the file navigator on the left side corresponds to the
System Preferences -> General -> Sidebar icon size
You can also override the system size inside Xcode using the
Xcode -> Preferences -> General -> Navigator Size