This is a playmat to enhance games of Dungeon Roll. (You can play the game without it.) It's printed on canvas so you can fold it and it should fit in the box.
This uses the official artwork created by Rob Lundy, the Art Lead for Dungeon Roll. I'm not connected to them, other than being a Kickstarter backer: however, the game's makers provided this artwork free for fans, and seem delighted that fans are ordering custom prints.
At time of writing, I haven't received my copy of this yet, so I don't know to what extent the edges will need hemming.