Tricky Screen Replacement (Please help!)

Hi everybody,

I'm currently trying to re-do a phone screen replacement for a University assignment.

I've followed endless tutorials, but keep encountering the same problem: the video I want to

use to replace the original, physical green screen is too big and appears stretched when I use Mocha and follow this tutorial:

https://www.youtube.com/watch?v=6CKTTkTLYWo&t=426s

However, when I try to precomp my video and change the dimensions to the size of a phone screen, Mocha doesn't like it because the replacement video must have exactly the same dimensions as the video with the phone in it. (Hope I'm making sense).

Hope someone can help me figure this out, I've been stuck with it for weeks.

Many thanks in advance!