There is no lawful size, the interior play length of billiards must be equal to twice its width. The surface of play constitutes thus a double perfect square.
The choice of a size of billiards is related to the characterics of the part where you plan to install it.
The dimension of the part influences over the size and the length of the cues to be used. The following table will help you to to establish the useful dimensions (the most favorables with the play being on green bottom).
