Does anyone know what the pixel aspect ratio is of the 5D Mark II's LCD? Doesn't look like it's either 1.33:1 (4:3) or 1.50 (3:2)...

DD.

It's a 640x480 screen therefore 4:3 aspect ratio.

How important is the actual size of the screen? When it displays video, its 16x9 letterboxed, not 4x3.

The size isn't that important. You can get a loupe that magnifies it's apparent size. Resolution is important though as it would be nice to see the final image, rather than large pixels. It's hard to focus on the fly with these things!