On page 117 it says to create a rectangle and change the Size value (width) to 680.
The composition is the NTSC DV preset which is 720 px wide.
However the rectangle is wider than the comp.
If I put the rectangle width to 720 it is definitely overlapping the width.
Why is this?
It's because of Pixel Aspect Ratio, a concept you'll need to grasp fairly early on:
An overwhelming majority of video is NOT composed of square-pixel images. In the case of an NTSC DV comp preset, the pixels are displayed a little narrower than they are tall. On the other hand, the pixels are displayed wider than they are tall in a DVCPro HD comp preset.
It can get a bit confusing. In your immediate case, it sounds like you made a solid with square pixels or wide pixels. You can change the solid's settings and use the drop-down menu to adjust the pixel aspect ratio.
Thanks very much Dave !