Not an expert here, but I'd say that YA length is shorter than the 700 page adult fantasy novel.
Here's my suggestion (What I do when I have a question like this) Go to Barnes and Nobles and take a peek at books of this Genre. I think if you can pinch more than an inche, you'll find that the younger they are, the shorter their attention spans are.
But most kids will read a longer book if they love the story, like LOTR, and Narnia.