The size of a go board has gone through many changes, from 9×9 to 15×15 to 17×17 and then finally to 19×19. Actually no matter what the size is, a proper game of Go can be played. It can even be enlarged to 21×21 or 23×23. Why then, is it that it stops at 19×19?
This is actually related to the idea of balance. As we all know, on a go board, the 3rd line is the territory line, while the 4th line is influence line. If we want to get all the points on the 3rd line and below, we need 56 stones, and get 136 points in total. If we want to get all the points in the centre on the 4th line and above, we need 48 stones, and can get 121 points in total.
56 stones to get 136 Points in the sides and corners
48 stones to get 121 points in the centre
Taking the average number of points per stone:
136/56 = 2.4286 (For the sides and corners)
121/48 = 2.5208 (For the centre)
The difference between the two values is 0.09 points, which is almost negligible. This means that the efficiency of the stones on the 3rd line and those on the 4th line are almost the same.
If the size of a go board gets bigger or smaller, it will lead to imbalance between territory and influence. The value of the centre will the different from that of the sides and corners.
That is the reason why the size of a board remains at 19×19.