FCPX - Opening and End Credits Issue

I am working on my first short film for festival submission in fcpx. 1920x1080
at 24fps. In the opening credits, I am looking for a guideline for text size. I am working on a 50 inch monitor and I don't have any reference from my screen to the theatre screen?

Also, the end credits, my text seems to be shaky in playback, is
that normal? My opening credits and video are fine, until we get to the end credits, it shakes and shimmers? I've deleted and redone them numerous times
and it's very confusing.

Need some assistance,
Thx
A.