Here's what he looked like originally:
As you can see, Withers was nearly as tall as my nelf.
Post-patch 5.0.4, he's been sized down significantly, and when summoned Withers will be about the same size as the Teldrassil Sproutling now. I would say he comes up to about my nelf's knees (standing).
When Withers gets dehydrated and thirsty, he becomes even tinier than usual. He comes up about to about mid-shin when next to my nelf (standing).
He's not bugged anymore, btw, and will drink from a nearby water source which will cause him to regrow to his summoned size. The emote for him drinking seems to have disappeared, though.
Is this an intentional change? I don't know, but I kind of adore him in this new super mini-size. :P