That looks a cool set 😃

These site smay be of interest mate - they convert inches to pixels for you (many covers seem to have a DPI of 300):-

https://www.ninjaunits.com/converters/pixels/pixels-inches/

http://www.nationsphotolab.com/pixelchart.aspx

A few people on here have bought old 3-disc boxsets from the local thrift/charity store (it doesn’t even have to have any discs in them etc) - and use that to take apart and learn how these box sets are made - as well as measuring dimensions etc. Some prefer 7mm slimline cases, or the 12mm spine cases - so sizes may vary depending upon preference.

Good luck with it!