Seth,
You don’t need to change the frame size in FCP, you change it your QuickTime Output Settings. Check out the article I wrote here:
https://www.kenstone.net/fcp_homepage/understanding_16_9.html
Scroll to the bottom.
If you want to change the Canvas size, use a codec that can handle varying comp sizes, like the Animation codec. However, that is not the route you really want to go. I use varying comp sizes all the time in FCP. Usually for oversized sequences. For that I use Animation or Microcosm from Digital Anarchy.
Kevin Monahan
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fcpworld.com