Have you seen the new Georges Jeanty interview (linked through Whedonesque)? He has this to say about Dawn:

"Joss ... actually pointed out, case in point, that he had the idea of Dawn becoming big when he was doing the series but he could never effectively do that and not make it look cheesy in some way; and now that we are doing the comic that was one of the first things he wanted to do, was to make her a giant because he has an arc for her that’s going to lead her in places that you don't even want to know yet."

Looks like it's not going to be a case of "oh, that's why she's big. Let's get her back to normal sizer again in the next episode" after all...