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.
Since you haven't provided any screenshots of your own work, no actual settings and so on we can't really tell you what you may be doing wrong. You need to be more specific and give us something to work it. You know, describing invisible elephants is not that useful.