Assuming your box is a perfect cube
You can divide the surface area by 6 for the 6 equal square faces of your box
100/6 = 16.67 ft2 per side of the box
√16.67= 4.08 ft maximum per side (but this is not a whole number)
4 x 4 x 4 cube works with each face being 4 x 4 squares
6(4 x 4) = 6(16) = 96 ft2
It could hold a volume of
43 = 64 ft3
If your box is rectangular 7 x 4 x 2 works for a surface area of 100 ft2 with a volume of 56 ft3
(7 x 4)2 = 56 ft2
(7 x 2)2 = 28 ft2
(4 x 2)2 = 16 ft2
56+ 16 + 28 = 100 ft2
You want to explore volume to surface area ratios approaching 1.
A closed surface area has to spread around and over a volume to contain it so surface area is usually larger.
I hope you find this information useful