I have different formats I`d like to edit into one movie (Iphone Pictures, Iphone movies, dv movies, avchd movies etc). I usually let fcpx decide project settings (first Clip added to timeline), but now I probably should concider customizing.
First, make a list of the resolution and frame sizes of all your sources.
Second, sit down and make a rational list of your most likely delivery target resolutions. (I say rational because in my experience, everyone imagines that their constructions deserve to end up on the big screen, tho in practice, a sliver of a sliver of a sliver of a percent of them actually ever do.
You want to create a native raster for your project that scales your asset clips the LEAST you can for your target output.
That may mean you try to find a middle ground between your highest output need and your lowest one. That way, some clips scale up and some clips scale down - but as few as possible have to scale from one end of the resolution spectrum to the other.
But you've got to think it through. One SD DV clip from an old shoot placed into a storyline of mostly 1920x1080 HD content needn't drag the entire project down to a lower rez. And it might be more effective to matte that shot into a smaller window or something to accommodate the smaller raster, rather than blowing it up and losing that much resolution.
No rules for this stuff, other than perhaps that the web has kinda standardized on 720p (1280x720) and most web sites seem to use this as the default starting place. And the benefit is that it's pretty near the middle of the raster size table.
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I agree with Bill. However I'd be tempted however to create your project, and drop in a clip with your highest resolution to create your project settings.
I'd edit away with this resolution, then you can always watch the final rendered edit and decide how you want to export it. If you decide that there is too much low resolution material, you can easily export your 1080p project as a 720p, or a 540p.
Don't live your life in a secondary storyline.