I thought it was 3500, but I could be wrong (feels like ages ago I wrote all those essays). My best advice, and the advice I've seen countless times here is:
1. ANSWER THE QUESTION. All too often applicants get passionate and veer in a direction that has nothing to do with the original prompt. Make sure everything that appears in the essay helps to answer the prompt in some way. The word limit makes each sentence that much more important.
2. Make your essay unique. Don't lie or embellish when it isn't warranted (that is bound to bite you in the butt come interview time), but really think about what makes you and your perspective yours and yours alone. A lot of people are going to have a similar story: an injury landed them or a loved one in PT and their lives were changed by a certain therapist who showed them that PT is more than just exercises. This is a great thing, and I am in no way undermining its significance (because I totally had this experience), but the essay should be more about you and your thought process, not another therapist's. Mention your observations, revelations you've had during the prerequisite process, and why PT is important to YOU. Just be honest and really take your time with it.