I have an issue with FCP where sometimes when I open a project my co-editor sends me the aspect ratio is off (this also happens when I copy and paste into a brand new project). This only happens on the archive footage we are using. If the footage is 4:3 it comes in at half that size and sometimes adds black bars on the side of footage that should be full screen. This only happens on my computer so I'm sure there's setting some where to fix this I just have no idea where it is.

Here are the aspect ratios I'm working in:
FCP project settings: 1440 x 1080
Interview Footage: 1440 x1080 (Mini DV)
Archive Footage: 1920 x 1080 (up-converted 4:3 footage)
Archive Footage: 4x3

* Why have you up-converted 4:3 to 1920x1080 -- which is 16:9? That would certainly have right and left side letterboxing.

* Are you working in an anamorphic timeline? If so, is the anamorphic box checked in the timeline settings?

* Is your pixel aspect ratio correct for the format?

As far as I can tell, here's how Final Cut "thinks" about aspect ratios:

It will almost always try and "fit" your footage into your Sequence. That means scaling a clip right up to the edges of the sequence, and distorting the Aspect Ratio of DV/Anamorphic/Square Pixel footage, so it matches those of the sequence. This results in 4x3 clips being pillarboxed into 16x9 sequences, and 16x9 clips being letterboxed into 4x3 sequences. FCP will also attempt to distort the footage so it appears in it's original ratio.

You will be able to see these changes in the Motion tab when you open a clip in the Viewer. Under Basic Motion, the Scale property may be automatically adjusted, as well as the Aspect Ratio property under Distort.

The fix is easy enough - just adjust your first clip until it's correct for the way you want it to appear, Copy the clip from the Timeline, then select the rest of the clips you want to fix, right-click, then choose "Paste Attributes" and choose "Basic Motion". This will apply the Basic Motion values from your fixed clip onto all the other clips you've selected.

You can right-click a clip in the Browser, select Item Properties > Format, then compare those Frame Size and Pixel Aspect values with your Sequence > Settings to get an idea of how FCP is trying to fit your clips into the sequence.