How can I increase the font size in WebStorm?

Change font size with MouseWheel

(called “zooming in the editor”)

To enable changing font size in the editor

1. Open the Settings/Preferences dialog, expand the Editor node, and
   click General.
2. Make sure that the setting Change font size (Zoom) with
   Ctrl+MouseWheel is enabled.

https://www.jetbrains.com/help/idea/2017.1/zooming-in-the-editor.html