OK - geometry saves the day again!! If you want the diagonal measurement, use the following formula:
A-squared + B-squared = c-squared
where A and B are the short sides of a triangle and C is the long diagonal side of the triangle.
So, in this case for 6" squares on point, the width would be calculated as: 6x6 + 6x6 = 72. The square root of 72 is 8.48528137. The width could be rounded up to 8.5.
You can get the square root on a calculator that has a square root button. Enter 72, then press the square root button which looks like √ or just google "square root of 72". It will give you the answer.