You can put this solution on YOUR website! A rectangular box is 6 inches what?
Then you say:
6 inches tall x 7 inches long
Is this a rectangular box with square ends? How does the box have to go in the circle?
Once you answer those questions, draw a line from one corner to the opposite corner and calculate the hypotenuse, that's the smallest diameter of the circle you need. And remember, if what you are trying to push through is a rectangle, the formula for the hypotenuse is a^2+b^2 = c^2.
Here's what you need if the ends are square and you push the box through end-on-end:
