Actually Law 2 of the Laws of Association Football states:
The ball is:
- Spherical
- Made of leather or other suitable material
- Of a circumference of not more than 70 cm (28 ins) and not less than 68 cm (27 ins)
- Of a pressure equal to 0.6 - 1.1 atmosphere (600 - 1,100 g/cm2) at sea level (8.5 lbs/sq in - 15.6 lbs/sq in)
Therefore for the ball to be fully over the line then it must have gone over by it's diameter. Since the circumfrance of the ball is equal to πD and therefore the diameter of a the ball is 8.51 to 8.94 inches over the line. Ask Mrs Mobile - half an inch IS important.