Let the side length of a square be x,
Volume: V=(8-2X)(5-2X)X
=40X-26X^2+4X^3
V'=40-52X+ 12X^2
Let V'=0,
40-52X+ 12X^2=0
(4X-4)(3X- 10)=0
4X-4=0, X 1= 1 (cm),
3X- 10=0, X2= 10/3 (cm), because the width direction is 5-2x = 15/3-20/3.
Therefore, when the side length of the small square is 1CM, the volume of the box is the largest.