From the sounds of it, it seems that "World's Largest Dungeons" might be what I need. Will it fit inside of minecraft though is the question?
Yes, it will fit. The reason it will fit is the same reason you shouldn't bother building it.
It's a single expansive flat dungeon. So you'll avoid the single biggest difficult in putting most dungeons into minecraft, which is that they often have lots of verticality, which eventually will overflow the 255 block limit you can build up. IIRC it's also pretty blocky, so you won't have to worry too much about corridors not running along north/south and east/west lines.
But just because it will fit doesn't mean you should build it. To make something worthwhile to build in minecraft, it needs to have some visual elegance to it to make it worth looking at, and while there are a few larger areas with lakes of lava and relatively high ceilings IIRC, there just isn't the dramatic interlocking 3D terrain you'd find in better done dungeon maps.
It's a very doable map, but you might not get a lot of nerd points and acclaim for doing it compared to more iconic settings like Temple of Elemental Evil, White Plume Mountain, or even the Caves of Chaos. It certainly wouldn't be my first choice to spend hundreds of hours building.
I'm pretty sure someone has already done Castle Ravenloft. I wouldn't be surprised if there was a Tomb of Horrors out there. You might want o look at what people have already done before deciding on a project.