Board Thread:Support Requests - Designing Your Wiki/@comment-32590715-20200715125514/@comment-23989026-20200715133336

You could use theme

As you quote width as 300px I assume you are using europa infobox. So in MediaWiki:Wikia.css add the different widths .pi-europa.pi-theme-400 { width: 400px; } .pi-europa.pi-theme-250 { width: 250px; }

You want a switch to pick widths so use theme-source which is included in the top row of the infobox 

Then use "width" variable when using the infobox, which pulls the width from MediaWiki:Wikia.css