Hey there! Let me try to help you here—and remember, if you end up at 650 or even 625 words, that is just fine. But you DO want to make sure you cover all your bases, and that will be hard to do in fewer than 625 words.
First, you might want to talk about specific courses that you want to take. Especially courses that might not necessarily be available at other schools.
Next, you might choose to do some research, and find some professors that are VERY well-regarded in your field of choice. Name that professor, or even two of them, and explain why you really, really want to meet with that person.
Next, you might want to talk about the Team Fuqua culture. This is one of the things that makes Duke very unique. How does it relate to you? Why do you want to be in such a collaborative atmosphere?
Next, you might choose to talk about the fact that you might be an international applicant, an you know that Duke is one of the most international of all programs.
Next, you might want to mention some clubs, or extra-curricular activities at Duke that could be very exciting for you to join.
Next, you might want to talk about why you would like to be in the southeast in the US. Any good reasons why?
And finally, you might be able to speak about any current students or alums who have met or spoken with you. Remember, by reachibg out to people, you will show that you are doing lots of good work, and researching the school. They want to know that you are taking the process seriously. That should cover about 650 or so words, wouldn’t you say? J