There actually a few different ways you can do this in FCP. But it really depends on what type of footage you're working with and the desired outcome.
For example, if you had footage on a 4:3 DV NTSC (720x480 CCIR 601) timeline and wanted to put a 16:9 mask over it, you could nest the sequence and apply a 16:9 widescreen filter to it (Effects -> Matte -> Widescreen).
Another example is if you want to crop out some of the image in a clip and fit it to the frame size. Here, you would double-click the clip and perform a Center/Scale adjustment on the image (these adjustments are in the Motion tab). Be careful with this though, because you WILL lose image quality on the scale enlargement. In still photography, this isn't as big of a deal per say because you have 8+ megapixels of information to work with on most consumer cameras. But video is much lower resolution by comparison.