I wouldn’t worry so much about this. Pick pieces of sufficient difficulty that show your ability, that don’t sound too much alike. I would also ask them if they want you to cue the best three minutes in each piece. Stanford is not a conservatory, but a high level academic institution with a BA in music, and where many musicians don’t major in music… They want candidates who can contribute to musical organizations on campus. The audition in performance is one factor among many factors, unlike a conservatory where audition is the most important thing. It’s called a supplement for a reason.