The original square of piece of paper, 
with the cutting and folding lines would look like this: 
You see 4 congruent squares cut out of the corners. 
If the length of the sides of those squares is 4 inches, 
then the height of the box will be 4 inches. 
Let x be the length (in inches) of the side of the square (the bottom of the box). 
The surface area of the bottom (in square inches) is x^2, 
and the volume of the box, calculated as area of the bottom times height, 
(in cubic inches) is 4x^2 
So 4x^2 = 784---> x^2 = 784/4 ---> x = sqrt 196 = 14. 
Answer: 
14 inches 
Hope this helps :)