Your essay must not be longer than 650 words, but it can be shorter. Anything over 500 words is fine, as long as your essay is thoughtful and complete.
If your essay is running short, add relevant details and descriptions that help the reader get a better glimpse of the situation and your personality. Do not add fluffy adjectives, redundancies, or grandiose closing statements such as "Your university will make my dreams come true." This is likely off the topic of your story, and well, it's not memorable because a zillion essays end this way.
If you need help brainstorming topics, revising, or finalizing your essay I'd be glad to help. I'm a published writer wot over ten years of experience tutoring college applicants as well as degrees from the University of Michigan and Harvard.