I was just looking at the map of Hogwarts at TLC (I think drawn by JKR?) and all it says on the east side, with the Forbidden Forest is "Forest extends." Is Hogwarts close enough to the coast that it might extend a few miles to the sea? Is there something on the other side of it? Could you get lost in there and come out in some alternate dimension on the other side?
This is partly a practical question--in terms of Aurors defending Hogwarts during HBP, it would seem reasonable to know the properties of the eastern boundary--but also just a curiosity. So many things could be in there, and I'd assume its size is magically enhanced at any rate. (Heck, maybe to Muggles, it's just a line of trees by the ruined castle!) But what's on the other side? Inquiring minds, and all that.
Personally, I'm all for making it up, but I don't know if there's canon to the contrary.