The wingspan of
Quetzalcoatlus northropi (the largest pterosaur) was from 33-36 feet in length:
http://en.wikipedia.org/wiki/File:Quetzscale1.png
The length of the creature's neck would give a shield-maid like Eowyn plenty to chop at. At an estimated top weight of perhaps 550 pounds (or 260 pounds, depending on the expert conjecturing), it would seem to be able to carry the physical manifestation of a Ringwraith (who, I assume, are not as weighty as your average flesh and blood mortal).
The wings are indeed batlike, but it is toothless, as it was a later Cretaceous version of a pterosaur. Earlier incarnations, like pterodactyls did indeed have teeth.
I think I prefer this version of a Fell Beast, as opposed to the stinky, plucked chicken variety.