The cardboard must be at least 25 in x 37 in.
Work Step by Step
As shown in the diagram, each side of the box is 16 in tall and 9 in wide. Each flap is half the width of the base, so each flap is 4.5 inches. The total width required for the cardboard must include 2 flaps and one side. $W=4.5+4.5+16=25\ in$ The total length for the cardboard must include 4 sides and a 1 inch glue flap. $L=(4*9)+1=36+1=37\ in$