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.
It is important to cover whatever period you intend to include in your personal statement because time spent planning your sequence and the method you feel happiest with will undoubtedly make the actual writing that much easier for you.