User blog comment:BertH/Fluid layout is arriving on December 4/@comment-168424-20131118222501

So, from what I've seen on WoWWiki, fluid layout is good for screens above about 1500px wide. For screens that are narrower (mostly old PCs and laptops), fluid layout is slightly worse.

From Wikia sources (don't ask me who): "For instance, the most standard resolution for users on Wikia is 1366px, representing about 25% of our total visits. With the fluid layout, users on that resolution will see the increased minimum article content space of about 700px (an increase of 30px from today)."

Unfortunately, on a 1366px wide screen, WoWWiki previously gave about 820px of content space, so fluid layout is a loss of about 100+px of content space. Remember, though, WoWWiki's extra wide fixed width skin was the exception.