The door opening is 10"w x 5" high. Height of 5" is about half the width of the box, then tapers to a bit over 3".
That's the opening size. The inside of the box is bigger... about 12 x 7, tapering on the end. I get an Aerostich tool roll in there and cram a few other things around it.
I have some to ship this week, then I'm off to the Overland Expo for a couple of weeks of riding.