The volume the carton can hold is 1000 units.
The formula for volume of a cube is length times width times height, and the length, width and height are all the same length, and the length is the 'cube root' of the carton or box or whatever. So, the answer is the carton is 10 inches wide, 10 inches long, and 10 inches high.
10 x 10 = 100 square inches
100 x 10 = 1000 cubic inches