Well, if you have your eye on something in particular and you want to be extra sure, there’s always the option to private message (or PM) the user who posted it.

I think most of the designs on this site are designed for 12 mm spine cases, as those are generally the most common size for 2 disc blu-ray sets.

But since there are many slightly different options for cases (here for example) the best thing to do would probably be to buy whatever sized case will work best for your particular project and have them in hand before you start printing artwork.

As you can see from J.D.'s work a few posts back, some do sets of three, four, or more discs per film, by including material beyond what is in the NJVC set alone.

Once you have whatever empty case you choose, you can cut trial and error pieces of scrap paper to double check the precise size your printed insert will need to be.

And if the size you need is slightly larger or smaller than the design you’ve picked by a few mm, use a site like this one to precisely resize photo:
(www.filesresizeimage.net)

or this one to precisely resize pdfs:
(www.sejda.com/resize-pdf).

But if you get a 12 mm case, like say this one, most of the designs here should already be all set to print as they are.