Hi there. I don't know if this post is too late or not, your post was on the first of August, and I just registered today, so
I did not really read your essay, but I counted the words in MS Word and the length is good. I mean anywhere near 500 words or more is good. I'm cramming for the Toefl myself and I always try to write at least about 500 - 600 wrods. No matter how you deliver your essay, more words means you have more to say and more chances for you to show off your vocabs. Well, at least that's what I think.
Did you time your essay? Remember you only have 30 minutes. I normally struggle in the first 10' minutes for the introduction (since I often change my topic sentences while I process) and then in the rest 25' I just churn out words like I have nothing left to lose lol