Goal: Use Final Cut Pro X 10.0.7 to produce a 640x1136 video (iPhone 5 Portrait). You can use these same steps to produce virtually any dimensions you want. My example is for the iPhone 5, in Portrait mode.
This information is available in various places, but you have to realy work to find it, so I thought I'd post a step-by-step guide here.
By the way, you might think you could just create a FCPX Project with your desired dimensions, but you can't (at least with 10.0.7). Projects only support certainly dimensions. But there are other ways, as the rest of this post describes:
1) Create an initial movie with the desired dimensions, to serve as a template. We'll call it "Template.mov". The only thing that matters is the dimensions. It can be in any codec FCPX understands, and virtually any length.
I'm using a 2 second video shot on my iPhone for the template. You could also create a blank image (.png, whatever) with the proper dimensions, copy and paste it into a new Quick Time Pro 7 window, and Export it using custom settings. There's lots of different ways to do this.
2) Drop Template.mov into a FCPX event.
3) Select Template.mov in the Event browser, and select File->Duplicate (or command-D). You probably could just edit "Template.mov", but it's a good idea to save it for reuse.
3) Change the name of the copied video. I'm using "iPhone Video".
4) Right-click "iPhone Video" and select "Open in Timeline".
5) If you made the Template video from a still image, you may need to change the duration (ctrl+D). You don't have to make it very long, but it needs to be longer than 1 frame. A few seconds should be enough.
6) Use this timeline like you'd use a project's timeline. Drop in new content, trim, transition, etc. There may be things you can't do in it, but I haven't found any yet. I'm guessing Chapters may not work, but I haven't tried them.
7) Note that when you drop new content into the timeline, it may size funny. That's because the default setting for "Spacial Conform" is "Fit". Simply select the newly-dropped content, and in the Video settings inspector, scroll down to "Spacial Conform" and change it. You could use "Fill", which fills the frame vertically, or "None", which uses the source's original height. Then crop/transform/whatever to get what you want.
8) When you're ready to render, select "iPhone Video" in the Event browser, then use the "Share" button. You want to use the "Export File" destination, which you may have to add. Pick your codec, and let 'er rip.
9) When the rendering is done, you should have a video in your desired dimensions.
That's all there is to it. Hope this helps someone.
PS: Don't ask why someone would want to create an iPod video using FCPX. The point is that you may need to create a video with non-standard dimensions at some point in time, and this is how to do it.
PPS: Most of this information came from http://library.creativecow.net/articles/payton_t/FCPX_Custom-Resolution-Timeline s/video-tutorial. I just wrote it down, while I tried to do what the video suggested.