Hello how may I help you? What was the biggest cloud ever recorded? Do you mean tallest or widest? Both. Single Cloud or cluster? Cluster. The largest cluster of of clouds was a cluster of rain clouds over china at 400 miles wide and 2000 feet tall. Wow. That is big. Why did it form? Yes, Is is the largest so why did it form? so big? There was a polar vortex pushing the clouds to cluster