For a case like this, the measurements of the finished cover insert would be approximately:
width: 10 7/16" by
height: 5 13/16"
However, remember that there are various case styles, (standard, slimline,etc). Make sure to measure for what you aquire specifically.
Also make sure to print with about an extra 1/16" or so on all sides to trim off. This is known as a “bleed” and will allow for clean cuts without any white from the paper at the edges.
This site may also be helpful.