Using the Pythagorean theorem, we have:
(diagonal)^2 = (length)^2 + (height)^2 + (width)^2
Simplifying this equation and plugging in the given values, we get:
20^2 = 7^2 + 11^2 + w^2
400 = 49 + 121 + w^2
400 = 170 + w^2
230 = w^2
w = sqrt(230)
w ≈ 15.165 feet
Therefore, the width of the box is approximately 15.165 feet.