Of the four neck, back and tail vertebrae discovered, together forming the holotype MPM 10002, the most impressive is the back vertebra measuring 1.06 meters (3.48 ft) tall and 1.68 meters (5.51 ft) wide. This is the broadest sauropod vertebra known, and two-thirds of its width is made up of the huge wing-like diapophyses (side processes which supported the ribs), which are heavily buttresed and merge with both the centrum and the neural spine, forming a wide spade-like shape (in most sauropods, like Argentinosaurus, they are far less large, lack buttresses, and form a simple cross-bar shape). The huge size of the diapophyses indicated a very wide rib cage, which would have been anywhere from 5 to 8 meters wide, which would make it not just one of the largest dinosaurs, but also possibly the widest and heaviest. The neck was also unusual; based on the single known neck vertebra, Puertasaurus had a very long, squat neck with wide cervical ribs and thick but short neural spines (unlike its relative Futalognkosaurus, whose neck was also very long, but was much deeper, with tall "shark fin" neural spines). This gave the neck a huge vertical range of motion and flexibility, and it may have been able to "bend over backwards" so that Puertasaurus could reach higher branches behind its head, without having to move its whole body - as a trade-off, the neck's sideways flexibility was more limited, though the extreme width of the neck may indicate a wide head as well, the jaws of which could take in more food per bite to make up for the lack of lateral flexibility. This strange, highly specialized squat-neck design is seen nowhere else in the Dinosauria.
Fernando Novas, one of the paleontologists who described Puertasaurus reuili, estimated that the new species was approximately 35 to 40 meters (115 to 131 ft) long and weighed between 80 and 100 metric tons (88 to 110 short tons). If correct, this would place it as one of the biggest dinosaurs (and the biggest titanosaur) ever to walk the earth, though rivaled in size by the more primitive Argentinosaurus. It would also extend the period of time in which such very large sauropods lived; this species roamed what is now Patagonia towards the end of the Cretaceous period, about 70 million years ago, in the early Maastrichtian. More recent reconstructions suggest a total length closer to 30 meters.