Rosh Fragger

Doom 2 back in 2005 I believe.

Far Cry series

I believe the very first mega PWAD that I played was Coopbuild.wad made by Toke. It was probably my first day at ZDaemon back in 2010, and I happened to join a server running Coopbuildlm + Supercoop2d + Skins. Good times they were... got a whole lot of nostalgic vibes just now.

Personally, I find megawads having long maps quite fun. But there is a distinction between making a map tedious, and making it long as well as fun to explore; giving the player a desire to progress further and let the map reveal itself. The creative aspect needs to be there, otherwise the long and stretched out maps can very easily give out an expression of repetitiveness, which can make a map pretty boring after a while. Megawads such as Eviternity, Struggle, Resurgence are a great example of how you can make the maps somewhat longer and still have most of the areas/rooms look and feel different from one another. So in the end, it all boils down to how the map is played out. A map doesn't necessarily have to be big for it to be fun after all. Some authors are able to create masterpieces by limiting themselves to not go beyond a certain map size, while others can make their maps seem almost open-world like, with no value-addition whatsoever.