So, I think that whoever runs the DPG website did a similar method that I did to come up with the Mosasaurus' size.
I had read the Mosasaurus card when it was revealed on their Facebook page, but I didn't really memorize it. The other day, I was really bothered back when JW was out, people said the Mosasaurus was astronomically huge (Especially claims that it was 667 feet long, which someone screwed up somewhere in their math).
Anyways, I was telling a friend, my best estimate for the Mosasaurus' size, using the Indominus rex as reference.
When the Indominus rex eats Nick, you can see that half his body is in the I. rex's mouth (with some space at the back of the jaws and of course the back of the head.)
Eric Edelstein is 6' 4'', so I estimated that the Indominus rex's head was roughly 6-7 feet long total.
When the Mosasaurus snatches the Indominus, while never given an exact side by side, reference, I could see that the Indominus rex's head length matched a good portion of the Mosasaurus's head, so the Mosasaurus' head was over 7 feet long.
I looked at a still of the Mosasaur eating the Pteranodon and Zara. Katie McGrath is 5' 6'', and since a person's arm span is 1/1 with a person's height, and using her exposed arm as a measuring tool, I concluded that the entire Mosasaur's head was roughly 10 feet long at the minimum, and twelve feet as the maximum.
I looked at this image:
It states that the Mosasaurus in the image has a 1.7 meter long head, and a total length of 15 meters. I applied the same proportions to the Jurassic World Mosasaur with my own measurements (using my minimum of 10 feet for the head, ergo 3 meters):
1.7/15 = 3/x
X = 26
Ergo, according to my measurements, the Jurassic World Mosasaurus is 26 meters long.
The Dinosaur Protection Group officially states that the Mosasaurus is 25.9 meters. (So, canonically, the Mosasaurus is roughly 85 feet (84.97 feet to be exact.))
I was off by a tenth of a meter. This may be coincidence, but I was pretty dang close.
Of course, this is still a monstrous size, but I was actually surprised how close I was to the canon material (Again, I did not completely read its bio card, so I was originally assuming I'd get something in the low 70s range.)