Although the dealer identified this as a dorsal vert, based on its narrowness, the rib facets emerging from the lower lateral portions of the centrum, and the fact that there are no chevrons present on the ventral to indicate it was a caudal (tail), I believe it to be a cervical (neck). Based on studies of (admittedly uncommon) known pliosaur specimens, I estimate the animal would have been the size of an average bull or large cow Orca: probably ~22 feet in length (7 meters) with a comparable mass of ~4 tons. I did a nice piece on P. macromerus
in late 2017 that shows possible upper size limits for this amazing predator; you can check it out here.
The image below of Liopleurodon – another, similar member of the pliosauridae – shows the possible location of the centrum, as well as proportions related to the skeleton as a whole. The animal’s neck would’ve been around .75 meters (30 inches) thick.