Editing/Projects/Font size in the editing window

The past
For historical reasons, MediaWiki allowed inconsistent browser-default styles to apply to. Until October 2017, it allowed the browser to set the font family. This decision created an inconsistent user experience when editors change web browsers, and also created other problems, especially for extensions that require font-size matching, such as the long-requested syntax highlighting (see T180678).

The issue was further complicated by the fact MediaWiki uses  (instead of the "monospace, monospace" hack), which applies extra font size rules in some browsers (Firefox) and some versions. The old font heights were (all in pixels):

Current status
The font size is changing to 13 pixels high for monospace (the most common font setting for editors). This will shrink the height of the characters, by a barely noticeable 2.5%, for editors using Chrome, IE, and Edge (about 76% of editors). Editors using Firefox on Windows or Macs (about 15%) will see no change. Editors using Safari or Chrome on OS X/mac OS (about 12%) will notice a significant increase, which should improve accessibility.

The biggest change will be seen by the few Firefox users who have changed their edit font to serif/sans-serif (see T170223)

This change will happen during the second week of January 2017.

Choose your own font size
Regardless of the default for your web browser or OS, you can override the font size to your liking using a snippet like this, added to your user style subpage – for example, global.css: Note that fractional values may not work well with wikitext syntax highlighting.