"Of course," said Moody, taking a swig from his flask. "Took a leaf out of your book, Potter. Summoned it from my office into the forest. He wasn't anywhere on there."
"So he did Disapparate?" said Ron.
"You can't Disapparate on the grounds, Ron!" said Hermione.
I'm using this as proof that the Forbidden Forest, or at least the far eastern (see this question) side of it, is part of the Hogwarts ground. Anyway, my question is where was the boundary of the Hogwarts grounds? I have no idea how big the forest is, but it doesn't really matter; how much of it was part of the Hogwarts grounds? The whole forest, or just part? How much?