I was wondering what the biggest cave ever found (or that you have found, I realize this can be subjective without maps) in vanilla Minecraft was; biggest meaning a single cave system, like the one here, which is the largest single cave that I found (seed -123775873255737467, 1.6.4 back to at least 1.5.1), and as the second image shows, caves like this are very rare (the world is about 4000x2000 blocks).
Rei's Minimap, default (1:1) zoom:
Unmined map of the entire world at slice 20 (stands out much better but still noticable at slice 60) to compare to other cave systems, located on the left side about 1/3 of the way down:
Screenshots (some taken at maximum FOV) and using night vision; many areas are so dense that they don't have any recognizable passages:
I say vanilla Minecraft since I've modded my game to produce bigger caves, including rare cave systems that are much larger than this one, like this, and I know of mods that can change cave generation (Fewer Ravines/Less Caves, which can be set to make more). Also, the seed only works in 1.6.4 and earlier because cave generation is different in the snapshots.