Don't see why they couldn't get as big as the universe itself. (*Cough*) The only limit is your imagination. If you can imagine them being that big, then there you go. No problem. They can be that big if you (for some reason) want them to be. If you can't imagine it, then in that case, no, I guess it won't be happening.
Seems like something my Crepuscule would do as a prank; just wake up one morning, hear noises outside, look out my window, and just see some naked gal 10 times the size of the Earth. And she'd be all like "Oh hey, morning Rizzy. <3"
And then she'd punch the Earth in the face because she's a tulpa like that.