Size and Weight:
About Pteranodon (Pterodactyl):
Despite what many people think, there was no single species of pterosaur called a "pterodactyl;" the "pterodactyloids" were technically a large suborder of avian reptiles that included such creatures as Pteranodon, Pterodactylus and the truly enormous Quetzalcoatlus, the largest winged creature in earth's history. (See a gallery of Pteranodon and Pterodactylus pictures and 10 Facts About "Pterodactyls.")
Still, if there's one specific pterosaur that folks have in mind when they say "pterodactyl," it's Pteranodon. This medium-sized, late Cretaceous pterosaur attained wingspans of about six feet, though its "wings" were made of skin rather than feathers; its other vaguely birdlike characteristics included (possibly) webbed feet and a toothless beak. Weirdly, the prominent, foot-long crest of Pteranodon was actually part of its skull--and may have functioned as a combination rudder and mating display. Pteranodon was only distantly related to prehistoric birds, which evolved not from pterosaurs but from small, feathered dinosaurs.
Paleontologists aren't certain exactly how, or how often, Pteranodon moved through the air. Most researchers believe this creature was primarily a glider, though it's not inconceivable that it actively flapped its wings every now and then, and the prominent crest on top of its head may have helped stabilize it during flight. There's also the possibility that Pteranodon took to the air only rarely, instead spending most of its time stalking the ground on two feet, like the contemporary raptors and tyrannosaurs of its North American habitat.