I can double check when I get home, but it should be the same dimensions as the inlay on a regular jewel case, except that there will be two of them. The double CD jewel case just used two inlays for the front and back sections. I hope that makes sense. It's just a traditional double jewel case like the ones used for Gran Tourismo 1 & 2, Resident Evil 2 or Metal Gear Solid ect.
If you need, I can double check the official measurements and post pictures this evening.