Keep It Short
As a general rule, you should keep it brief. You should be able to fully provide the admissions committee with a unique perspective as to who you are as a person in two pages or less. Graduate school admissions committees review hundreds, if not thousands, of applications. Writing a personal statement that is too lengthy may cause some member of the committee to become weary of your application.
Exceptions to the Rule
In the situation where there have been extenuating health or family circumstances, the personal statements is the place for you to address these issues. Admissions committees will want to know if there was a situation that caused your grades or GRE score to be lower than normal. This is the place to respectfully make the committee aware.
Make your graduate school personal statement a full and comprehensive expression of yourself. Seek to inform the admissions committee why you are a perfect candidate for admission, but within tight constraints. According to Career Center, "It often should be limited in length, no more than two pages or less. In some instances it may be longer, depending on the school's instructions."