Size and Weight:
As far as paleontologists can tell, Proconsul marks the time in primate evolution when the "old world" monkeys and apes diverged from a common ancestor--which means, in layman's terms, that Proconsul may (or may not) have been the first true ape. In fact, this ancient primate combined various characteristics of monkeys and apes; its hands and feet were more flexible than those of contemporary monkeys, but it still walked in a monkey-like way, parallel to the ground. Perhaps most tellingly, the various species of Proconsul (which ranged from a smallish 30 pounds or so to a largish 100) lacked tails, a distinctly ape-like trait. If Proconsul was, in fact, an ape, that would make it directly ancestral to humans, and perhaps even a true "hominid," though its brain size indicates that it wasn't much smarter than the average monkey.
However it winds up being classified, Proconsul holds a special place in hominid paleontology. When its remains were first discovered, back in 1909, Proconsul was not only the oldest ape yet identified, but the first prehistoric mammal ever to be unearthed in sub-Saharan Africa. The name "Proconsul" is a story in itself: this early Miocene primate wasn't named after the revered proconsuls (provincial governors) of ancient Rome, but after a pair of popular circus chimpanzees, both named Consul, one of which performed in England and the other in France. "Before Consul," as the Greek name translates, may not seem very dignified for such a remote human ancestor, but that's the moniker that has stuck!