Well, honestly, there is no word count feature available during the test and the time goes pretty fast - you aren't going to want to waste time counting words! I'd say the other posters are right. You need three paragraphs minimum to address the three tasks of the essay. I would just focus on trying to write something in that time that is as well developed as possible, that makes sense, and has as few grammar, spelling, punctuation errors as possible.
From everything that I read and hear, I don't think anyone, even the med schools, give much weight to the writing sample score. I am convinced it is in there just to add an extra hour of time to try to wear us down in between PS and BS.