As mentioned, this part of your application can be used to decide whether a school wants to invite you for an interview or not. Schools only accept students who have been invited for interviews, so getting to this step is a huge part of the process.
It is important to put yourself in the shoes of the Admissions Officer: why would your personal statement be unique, how are you different from the other candidates? Primarily you need to decide how you will divide your personal statement into digestible chucks. The most obvious way is to divide into paragraphs of 100 words. If you aim for between 500 and 600 words at most, it will help your admission officer when they have to read thousands of personal statements to find the perfect student.