The IJN Shinano was a huge aircraft carrier built by the Imperial Japanese Navy near the end of the war. It was huge because it was originally the 3rd Yamato Class Battleship, but was converted to be an aircraft carrier mid-construction.
There were 3 different ideas for the design of the ship. The IJN command eventually settled on a compromise design which made it basically a Support Carrier. It would have about 47 aircraft for self defense and carry a bunch of supplies for other Carriers. So they could stay at sea longer without having to return to port.
However, as soon as the Shinano left port to travel to another port it was spotted and sunk by the USS Archerfish, an American submarine. The reason it was sunk so easily is because the ship wasn't finished and I think the internal bulkheads weren't completed. To this day it's the biggest ship ever sunk by a submarine.
It was no doubt a devastating loss to the IJN. But if it had avoided being sunk, what kind of risk would it have posed to the USN?
Practically none. At that point in the war, the 50 planes or so that it would have used would have been swamped by the usual carrier task forces the US Navy brought to any battle. As comparison, almost 400 planes attacked the Yamato during Operation Ten-Go. Even if you give her credit for the 120 or so planes she could have carried in her role as support carrier, that's still a huge mismatch.
Yes, the Shinano was very large and it would have taken a lot of killing... but at the same time, what the Archerfish did to it is telling. She was basically a single target when the Japanese needed a lot.