Overall size is almost 6-1/4" x 6-1/4". The large opening is truly 6" x 4"; the small openings are bit under 2" x 2". As you can see in the photos, you can use it with the cubbies on the side or the top, too.
Also shown with a Mouse & Pumpkin Border and the Small Cinderella Silhouettes Set.
Laser-cut heavy chipboard. Ships flat but a breeze to put together with the slots and tabs precut (you'll just need to secure with a bit of white glue).
Design Team Member Rhonda Secrist Thomas made a Cinderella Shadowbox (see the wide photo below). More info in the blog. See her Supply List.