The short Box is a small dice Box that can fit up to 6 D6 dice up to 16,5mm (0.65in) large. I recommend dice that are 16mm (0.63in) large, so there is a bit of wiggle room.
The long Box is a big Box that can fit up to 10 D6 dice up to 15mm (0.59in) large. For me that were the dice to my Heroquest game, but they might be different in your game, so measure twice print once. The long box might also fit a few screws o something.