I found this 2009 screenshot from the German Wikipedia. I couldn't help but notice "Diese Seite ist 66 kB groß.", which indicates that the page has about 66 thousand characters.
I looked today and the indicator appears missing. But the feature may still exist in MediaWiki. I could not find a documentation of it on this site.
Is there any way to enable the size (kilobyte) indicator which shows above the edit field for all users of my MediaWiki installation? What do I need to change in LocalSettings.php?