I'm pretty sure it's not 'graded' mostly on length, merely a correlation between length and better grades. I read the Princeton Review 2010 and I remember them saying something similar. I'm sure it's because if you're better prepared, you're going to be capable of writing much more. So in that sense, length = better score. I wouldn't worry at all about length, maybe focus more on amount of paragraphs, which should be about 5.