You’re referring to the Editor mode with “Convert styled text to HTML” selected, right?
If you increase the font size there this will be reflected in the actual listing (in Preview mode) so I guess your request would require some kind of zoom option (75%, 100%, 150%, 200%,…).
this is a screen cap of the whole GS app window in the mode we use
On the iMac, this window is tiny, and there is no way to increase the font size in the Editor Pane. In atom.io or other text editors on this computer, we need to use 18pt-36pt font sizes for standard programming work.