I really think there is no right or wrong when it comes to length.  I know a lot of mine are long (not as long as I used to write on other sites though) but I have a lot to say!  I'm a lengthy person :)  However I've written 400 word reviews that covered everything I wanted to say.

What it comes down to for me is this, write as much as you want to cover the points that you think are important.  Don't go padding it just because you think it's short, and don't go deleting stuff just because you think it is long.  Finally, don't try to make all your reviews a certain length.  I just write until I've said it all and that's it.  I wouldn't set any mins or maxs on my reviews.