In terms of the amount of data, it's CEZ2. In 2.2.2, it has about 43000 sidedefs, which is 65% of the sidedef limit. In 2.2.0, before we removed some of the unused and unfinished sections, it was closer to 50000 sidedefs, or about 75% of the overall limit. So in a very real sense, it's not possible to make a map much larger than CEZ2.
In terms of the largest area covered, it's AGZ. Its bounding box is about 45000x35000 fracunits. The upper limit is 65536x65536. It's not nearly as dense as CEZ2 though, with only about 25000 sidedefs.
Other very large maps include THZ2 (~29000 sidedefs) and ERZ2 (~34000 sidedefs).
The 24h collab maps are not that large. If they were combined into one map, that would be about as large as ERZ2.