I am far from asserting that these dimensions will [Absolutely] prove to be correct if ever an individual should fall into the hands of men, but I am sure that they are approximately correct .
Perhaps you will in no case admit the possibility of the existence of an animal of 250 feet [in length]! Well, I leave it to you to fix yourself the utmost possible length of our Sea-serpent!"
[By Oudemans' count, the number of sightings estimating a total length of over 100 feet is around 10% of the total number of reports, and of course that '100 feet' now has to be reduced for some length of tail. Dropping off that top 10% of size estimates makes the largest legitimate size range of sightings equivalent to the HMS Daedalus Sea-serpent and in fact the text does state exactly that]
What I think has happened (And as a matter of fact, what I think I can prove about what happened) is that the largest reports that Oudemans tallies refer mainly to mistaken views of big whales, taking them for Sea-serpents, either singly or several in a line at once. In the instance above at far right, the actual sighting taken to be the midsection of a Sea-serpent is actually a sighting of the whale, and the extended lengths of head and tai are imaginary. The two figures in the center are Oudemans' reconstruction drawing for the Sea-serpents and the whale figure is repeated at far left purely for scale. This is a humpbacked whale.
In the case of the largest Sea-serpent tabulated, Oudemans clearly states above the table that the measurements are from the HMS Osborne Sea serpent sighting. The blog entry dealing with the HMS Osborne sighting is the earlier one on " VOID Sea Serpent categories", and the link to the discussion is below: