Haha, we did those of course, and I remember everyone stressed about them way more than they needed to. Length ranged from about 2 sentences to about a page. I'd say that one or two paragraphs is plenty.
What people talked about varied a lot. People often mentioned their family, where they are from, where they went to school, cool animal experiences they've had, how long they have wanted to be a vet, maybe what kind of vet they want to be, etc. As to if it was personal or professional, that just depends on you and how comfortable you are with sharing. Write something you don't mind the entire school knowing (including faculty), but don't stress about it too much. There is no grade. And the format is completely optional, you could do bullet points if you wanted.
No need to worry (or panic).