When submitting a video on Slideroom, are there any excerpts or any requirements on what to play? Is it just supposed a showcase of talent?
You can ask on the excellent music forum here on CC. You can also ask Harvard admissions. We did, and out of consideration my kid cued the best three minutes for them to listen to.
They are interested in how a student’s talent might contribute to the undergrad community as a whole, in this case in music 
Once you create your account and log in to Slideroom link that Harvard provides, there should be (if my memory serves me correctly) a set of instructions. If the instruction is general in nature, then my advise is not to exceed more than 5 minutes in length. Whatever you do submit should be exceptional.