What size of box would fit in the A4 paper?

An A4 paper size measures 210 mm x 297 mm (about 8.27 inches x 11.69 inches). To fit within an A4 paper, the box would need to have dimensions that are smaller than these measurements. A suitable box size could be approximately 200 mm x 280 mm x any height up to 10 mm, ensuring it fits comfortably within the A4 dimensions.