What are the dimensions of LEGO 10221 Super Star Destroyer? What is the length, width, and height of the 10221 Super Star Destroyer set, fully built, including its stand? 

I'm trying to make a display case for one.
So the box says 49 inches long, the official description says "under 50 inches long".
I just measured the width (about 13.25 inches) and height (about 9.5 inches) on my assembled set.

So a rough minimum inner size box would be 50"L x 13.5"W x 10"H.