Hmmm... "All ships shown at constant scale."
Scaling the Cobra in the image up to its BFG length of 1.25" makes the battle barge 7.25" long (2 1/8" longer than the BFG model) and makes the Iron Blood 16" long! Scaling up to real life at 1:56,000 gives a length of 24,086 meters or just over 24 kms. Seems a mite large for both ships...
If we scale the image to the length of a BFG battle barge (5.125"), the Iron Blood drops to 11.375", making it 16,333 meters (16 1/3 kms).