Check out similar books to yours in your local bookstore. See how long they are, and see who published them.
My next book is historical/fiction, and I've been capped by my publisher at 30,000 words. My last book (not historical) was 56,000 words, and I was asked to make it's prequel shorter.
There are YA books at 30,000 words and there are YA books much, much longer. I wouldn't worry about length so much as just getting it written. You can always edit out what isn't working, and go from there.
Best of luck to you! (If actors say "Break a leg," should writers say "Break your pen?")
Canada James