Roy P. MacKal’s A Living Dinosaur: The Search for Mokele-Mbembe (1987) at least provided the correct measurements and a full, if not absolutely faithful, quotation. By contrast, Michael Newton’s account in Hidden Animals (2009) is dependent upon MacKal (an acknowledged source) but garbles the discussion and claims Proyart described “tribal stories of a beast known as mokele-mbembe…” (p.44), which he certainly did not do.
Since this passage is almost never given in full and sometimes given incorrectly, allow me provide Proyart's original French and my translation to clarify things:
Les Missionnaires ont observé, en passant le long d’une forêt, la piste d’un animal qu’ils n’ont pas vu; mais qui doit être monstrueux: les traces de ses griffes s’appercevoient fur la terre, & y formoient une empreinte d’environ trois pieds de circonférence. En observant la disposition de ses pas, on a reconnu qu’il ne couroit pas dans cet endroit de son passage, & qu’il portoit ses pattes à la distance de sept à huit pieds les unes des autres.
The Missionaries have observed, passing along a forest, the trail of an animal they have not seen but which must be monstrous: the marks of his claws were noted upon the earth, and these composed a footprint of about three feet in circumference. By observing the disposition of his footsteps, it was recognized that he was not running in his passage, and he carried his legs at the distance of seven to eight feet apart.
Source: Liévin-Bonaventure Proyart, Histoire de Loango, Kakongo, et autres royaumes d’Afrique (Paris, 1776), 38-39.
The trouble seems to be that modern writers are confusing circumference for length (they are not the same), and they have taken Proyart’s adjective monstrueux as an indication that he was referring to a “monster.” However, while monstrueux carries the implication of “horrific” or “monstrous” today, in the eighteenth century, the word carried instead the connotation of “prodigious,” or very large, according to French dictionaries published in that era. Proyart and his sources did not express any great terror at the creature, whose description he sandwiches between passages on the elephant and the lion, and the context of the passage clearly implies that he considered the creature simply one of many animals of similar bulk.
A rhinoceros, for comparison, has a footprint averaging seven inches in diameter, according to zoologists, but which can easily top eleven inches. This yields a circumference (using pi times diameter) of two feet for an average rhino and our required three feet for the eleven-inch rhino foot. The rhinoceros also has a head-and-body length averaging around twelve feet, with a body length of about eight feet, meaning its legs are seven to eight feet apart.
The greatest objection to identifying Proyart’s monster as a rhinoceros is that the rhinoceros does not have claws. Its footprint, however, takes the appearance of three enormous, sometimes pointed, claw marks, which are actually its toes but appear distorted when smeared through mud. This can be seen in the footprints given below:
A second objection to identifying Proyart’s print as a hippopotamus or rhinoceros is the claim that such creatures do not currently live in the area of the Congo now home to the legend of mokele-mbembe. Proyart did not provide a location for his missionary friends’ sighting of the prints, so this objection cannot be sustained. There is no way to localize Proyart’s description to the same territory where the (current) mokele-mbembe myth is centered. Both the rhinoceros and the hippopotamus had ranges that included areas visited by missionaries when Proyart was active in Africa. The hippopotamus range included the Congo, and the rhinoceros the areas to the north. (Sadly, both have declined markedly since then and are no longer found in their historic range.)
Since Proyart describes the monster in a passage listing elephants and lions, this would imply that the creature was probably not in the rainforest, perhaps making the rhinoceros is the more likely animal.
At any rate, given the measurements provided by Proyart and the secondhand nature of the report, it is not possible to infer the existence of a dinosaur from his description of relatively small footprints.