That's truely not a flip answer. Just write it and worry about length and chapters later. Quick is good for chapters. Deciding where your chapters are and thier length is as personal a preference as writing it in the first place. Some authors have 2-3 page chapters, others are in the 10-15 page range. Setting, pacing and what needs to be in the scene should ultimately dictate the length.

If you are trying to write a novel and when you've finished can't seem to get to the 70K -120K mark, ask yourself if your story has enough depth for it.

Then this last summer, I ran off fifty thousand words without ever getting close to any resolution. Nobody could be more surprised than I was---the idea wasn't even in my head until the day before I started it. There's more to go---now I worry about keeping up production levels. (I took a break at the beginning of this month, to try to get some perspective and make a few notes---I've done neither, and plan shortly to just take up where I left off and hope for the best.)

So just write, and let things find their own length. Setting a target isn't a bad idea---but be prepared to junk your targets under the right circumstances.

[edited 'cause I forgot to put in the third paragraph of semi-solid advice---plus I added a couple of sentences and words.]

If it makes you guys feel any better, my fiction used to top out at 3000 words max. I'm discovering that with experience, it takes me longer to say what I have to say. This happens for a number of reasons.

1. I have learned to show, not tell.
2. I get deep into the characters heads.
3. I write more effective description.

My advice would be to complete your first draft and see how it turns out. Rewrites are great things for adding words, particularly if you are the type of writer who writes sparsely instead of padding your prose to begin with.

I also struggle with length, but I'm getting a lot better over time.

You are lucky in that YA's don't have to be terribly long. You can get away with as short as 40k, which really shouldn't be too hard to hit. Then, let it sit for a bit. Go back once the story isn't fresh in your mind, and try to read it as if you weren't the author. Can you still see everything vividly, from the words on the page? Chances are you'll need to add some descriptive prose in places if you've been struggling with 'white room syndrome'. Then, once you've combed over and fixed that draft to the best of your ability, let some readers crit it for you. They'll point out lots of other places that may need changes. By the time your novel has gone through a few re-writes, chances are good that the length will more closely resemble a novel.

I have a writer friend who has been so diagnosed, and when she shared what she'd learned about herself to qualify, several of our mutual friends indicated that they thought they might qualify as well.