I hope you made the distinction between "their" and "there" in your PS... sorry, this is a huge pet peeve of mine... this is a nuance of English grammar that we all learned back in grade school and yet, people who are applying to professional schools still constantly make this mistake in their writing.
To answer your question... I had about 3,700 characters in my PS. I wouldn't be concerned about how many characters you have, as long as you get your point across. I think most people will try to max out the 4,500 character limit, but it's your statement, so make it what you want...