First note that your essay will be graded by an e-rater, which is software that checks your essay for structural keywords and overall organization. Then it will be graded by a human grader who has about 2 minutes to read each essay.
According to Princeton Review "Cracking the GMAT," more length is better to get a high score from the e-rater (software that ). However 800score suggests that going on and on will irritate the human grader. I have read in a number of places that 300-500 words is a good length.
In total, my take is that you should try to get as much length as possible without rambling. Better to have a 350 word essay with good structure than a 600 word essay that repeats points or is poorly organized.