MediaWiki:Common.css

Note: After publishing, you may have to bypass your browser's cache to see the changes.

• Firefox / Safari: Hold Shift while clicking Reload, or press either Ctrl-F5 or Ctrl-R (⌘-R on a Mac)
• Google Chrome: Press Ctrl-Shift-R (⌘-Shift-R on a Mac)
• Internet Explorer / Edge: Hold Ctrl while clicking Refresh, or press Ctrl-F5
• Opera: Press Ctrl-F5.

/* CSS placed here will be applied to all skins */

.tinymce body,.tinymce td,.tinymce pre,.tinymce p,.tinymce div {font-size:16px;}
I added the line below as well;
.tinymce .mceContentBody p {   font-size: 1.2em !important; }/* FF requires a important here */