Zoom in the editor
The default font size for all documents is set on the Editor | Font page of the IDE settings Ctrl+Alt+S.
If necessary, you can change font size (zoom) in the active editor, and then reset font size to the default value. These operations apply to the active editor tab only.
Zoom with mouse
Press Ctrl+Alt+S to open the IDE settings and select Editor | General.
Make sure that the setting Change font size (Zoom) with Ctrl+MouseWheel is enabled.
Place the caret in the editor.
While keeping the Ctrl key pressed, rotate the mouse wheel. As you rotate the mouse wheel forward, font size grows larger; as you rotate the mouse wheel backwards, font size decreases.
The macOS users can use the trackpad "Pinch-to-Zoom" gesture to change the size of the font and the whole editing area.
Zoom with keyboard
Press Ctrl+Shift+A.
In the popup frame, start typing Increase font size or Decrease font size, and press Enter as soon as the corresponding command gets the focus.
The font will grow larger or smaller.
Restore default font size
Press Ctrl+Shift+A.
In the popup frame, start typing Reset font size and press Enter as soon as the corresponding command gets the focus.
The default font size will be restored.