Special:LongPages seems to support only pages in the NS:0 namespace. It'd be good if it supported all namespaces and we could also sort them by namespaces as we do with, ie, Special:UnconnectedPages.
You can find Special:UnconnectedPages
at https://phabricator.wikimedia.org/diffusion/EWBA/browse/master/client/includes/Specials/SpecialUnconnectedPages.php, It is Part of Wikibase.
Last Night I played with Special:LongPages. See P6670, You need to write Callback and Namespace Filter Functions.
I also have the need to search for the longest pages in other namespaces. For example on Wikidata it would be good to know which are the biggest properities and lexemes.
On Wikipedia it would be interesting to see which Help-pages are the longest to split/improve/consolidate them for making it easier for beginners.