If I recall correctly, I did that too--complete sentences, although it will be tough to be concise. But I think concise sentences would be good. Best of luck! 🙂
Just write it out in Word and click Tools -> Word Count... and it will give you the # of characters with spaces. That's the # AMCAS is counting. However, every <enter> is 2 characters or something like that, so don't forget to account for it.