IME, YA tends to be a little shorter, a little snappier, a little more to the point. It's one reason why I like YA.
But if it's a YA EPIC FANTASY, then it may be longer.
My advice: write the story, worry about word count after. If you've got a clear beginning and ending, with a fun ride from one to the other, people won't care too much if you're a little short or a little over.