Me, I started thinking of them as normal-sized human beings. Maybe none that are particularly tall, probably few grow over five feet high.
"But," some Baum purists will point out, "Baum says the Munchkins are as tall as Dorothy!"
Is that what he says? Way back in The Wonderful Wizard of Oz, we get a description of the very first people Dorothy meets in Oz:
They were not as big as the grown folk she had always been used to; but neither were they very small. In fact, they seemed about as tall as Dorothy, who was a well-grown child for her age, although they were, so far as looks go, many years older.
So, what do I say? Baum writes that these Munchkins were as tall Dorothy. Not that they all were. It is also noted that they were old, and it's well known that many people do get shorter with age. J.M. Barrie in his classic Peter and Wendy describes aging in a beautiful manner:
As you look at Wendy you may see her hair becoming white, and her figure little again, for all this happened long ago.
And in addition, some people just don't grow to be tall. I work with a woman who is scarcely four feet high.
John R. Neill's Munchkins were not abnormally short, and the Munchkins in Eric Shanower's The Blue Witch of Oz appear to be of normal size. W.W. Denslow drew the Munchkins at a short size, but he also drew almost everyone else in Oz at that size as well, including the Wizard, and excluding Glinda and the Soldier with the Green Whiskers.
So, that's about the size of it. "It" being the Munchkins. Or how I see it. You're welcome to disagree, and I'd love hearing your viewpoint.
And no, I definitely don't count the pint-sized Munchkins from the MGM movie as an example.