The length of your review is unimportant.
I cannot stress this enough. In the past, some people have gotten to the point where they've felt that if their review doesn't top out over at least one thousand words that it is crap or something. If you believe this, please stop. It's not true.
Your review should have constructive feedback, and that's all it needs. There doesn't need to be a novel of constructive feedback on a story, and it doesn't need to be less than three hundred words. It just needs to be good feedback.
On the RAC, we've even found that some of the long reviews are not as good as the shorter reviews, and we found a couple of trends that went along with that. First, an excessive amount of quotes. Quotes are absolutely wonderful in a review, but keep it within reason. Your review shouldn't be like 75% quotes. We're not going to give you a number of what we think it should be, because you all are smart, but yes. Definitely not 75%. -wink- And the second thing we've found is people filling their reviews with all kinds of... unnecessaries. When you write a review, remember that this is a review, not a story comment on LJ. You certainly don't need to be stuffy and over-professional in a review, but keep in mind that your goal is to help improve an author's writing by giving constructive feedback.
Phrases like, "Sorry if I ramble too much, it's really late and I'm really hyper/tired," or things that resemble that should not find a way into your review. Also, prefacing your review with, "I don't know if this will make sense, it's really late," etc? Also not the best.
Basically, before you submit your review, read over it. If there are things you want to say to help the author, say them. Go ahead and praise them. If you're giving suggestions, give them, and don't ramble off into waffling because you feel you might be too mean. Often we review our friends, and of course we heart them, but maybe don't devote half your review to things that aren't even related to the story.
We're not meaning to sound like we want your reviews to be boring. We want you to be succinct, but it's perfectly acceptable and entirely possible to write a SPEW review that doesn't require someone to scroll down on MNFF's review pages to read it. It's so possible that we wanted to show you! Here are some links to some of the great reviews you guys have written since December that are of a moderate to short length: