Here's a post on this subject I made in another thread last year:
Here are some wordcounts for first YA novels published since 2004:
(Stats from the Renaissance Learning site). Genre in brackets, as far as I can tell for those I've not read:
Cassandra Clare, City of Bones - 130, 949 words (fantasy)
Jenny Downham, Before I Die - 69,548 words (contemporary drama)
Siobhan Dowd, A Swift Pure Cry - 63,954 words (drama set in recent past)
Anthony McGowan, Hellbent - 66,824 words (horror/black comedy)
Melissa Marr, Wicked Lovely - 73,426 words (fantasy)
Stephenie Meyer, Twilight - 118,975 words (fantasy romance)
Simon Morden, The Lost Art - 115,175 words (SF) [had a novella published previously]
Robert Muchamore, The Recruit - 73,689 words (spy thriller)
Patrick Ness - The Knife of Never Letting Go - 112,022 words (SF) [had written adult novels previously]
Peadar O Guilin, The Inferior - 98,774 words (SF) [had short story sales previously]
Meg Rosoff, How I Live Now - 46,920 words (near-future borderline SF)
Jenny Valentine, Finding Violet Park [aka Me, The Missing and the Dead] - 36,244 words (contemporary drama/comedy)
That'll do for now - no doubt there are others which could be added to this list. That's a random trawl through as many debut novels in the last four years I can think of. Like them or hate them (and I haven't read them all) they are *debut* novels, which had to impress an editor and agent when the author was an unknown. I don't know what that shows, except that longer wordcounts are perfectly fine if that's what you need to tell the story (though that's clearly subjective but then so is everything else in writing), and YA novels can be as long as adult novels. On the other hand, they can be shorter than adult novels if needs be too.
Leaving Harry Potter aside, the longest YA novel I've come across is Aidan Chambers's This Is All, published in 2006 - 256,600 words, about a thousand shorter than Order of the Phoenix.