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.
Wouldn’t it be best if the “bleed” was already designed in the artwork of the file? i.e. you can put some cut markings on outside of the design, so you can use a ruler/cutter to cut it out, but that the artwork should spill over a little on the cut markings.
I’m going to have to look at the artwork in a program to see if Phil made the artwork a little oversized for bleed.
If not, then I may slightly enlarge the artwork to scale, and add cut line markings outside the design. The nice thing is he has edible versions, which I assume the text is vector. And should slightly enlarging the artwork won’t cause the text to go a little blurry.