The pitch is defined in Law 7 as being a rectangle of 22 yards/20.12m length and 10 feet / 3.05m in width.
Law 9 defines the creases and in the case of the return crease the minimum length of the crease which is 8ft /2.44 m behind the popping crease (the front crease) and therefore it must extend 4ft / 1.22 m behind the stumps.
This is to aid with judgement of back foot no balls (the bowlers back foot must land within and not touching the return crease) Law 24 5a.
As with all creases though they actually extend to the boundary behind the stumps.
The area that is cut short is a minimum of the pitch but tradition is to mow short back to the edge of the square.