Measure across the tips of the front claws for a width measurement. Add your previous length measurement to the width measurement, divide by 2. The result will be the average or squared measurement.
For example: length is 6'5", width is 6'3", total is 12'8" divided by 2 = 6'4"
That makes it square at 6'4".
Don't overly stretch it in either direction for a true measurement.