The giant flag of the french ship Le Généreux, which was captured by one of Admiral Nelson’s captains Sir Edward Berry at the Battle of Malta Convoy in 1800
The masts on these ships were over a hundred feet tall, so from the top you could see for about 12 miles. To communicate their intentions at that distance they needed really huge flags, especially because they might be hull-down - their hull is hidden over the horizon but not the masts - which would mean recognizing the ship was hard.