Explicit estimates of the entire size of the animal are rare; in 2010 Gregory S. Paul estimated the length at 2.5 meters, the weight at forty-five kilograms. William Parks found the hindlimb of his T. warreni to be about the same length overall as that of Thescelosaurus neglectus (93.0 centimeters (3.05 ft) for T. warreni versus 95.5 centimeters (3.13 ft) for T. neglectus), even though the shin was shorter than the thigh in T. neglectus, the opposite of T. warreni. Thus, the animal would have been comparable to the better-known Thescelosaurus in linear dimensions, despite proportional differences (around 1 meter (3.3 ft) tall at the hips, 2-2.5 meters (6.56-8.2 ft) long). The proportional differences probably would have made it lighter, though, as less weight was concentrated near the thigh. Like Thescelosaurus, it had thin partly ossified cartilaginous (intercostal) plates along the ribs. The shoulder girdle was robust. Parksosaurus had at least eighteen teeth in the maxilla and about twenty in the lower jaw; the number of teeth in the premaxilla is unknown.
Parksosaurus has been considered to be a hypsilophodont since its description. Recent reviews have dealt with it with little comment, although David B. Norman and colleagues (2004), in the framework of a paraphyletic Hypsilophodontidae, found it to be the sister taxon to Thescelosaurus, and Richard Butler and colleagues (2008) found that it may be close to the South American genus Gasparinisaura. However, basal ornithopod phylogeny is poorly known at this point, albeit under study. Like Thescelosaurus, Parksosaurus had a relatively robust hindlimb, and an elongate skull without as much of an arched shape to the forehead compared to other hypsilophodonts.