There is no rigid rule. But there are general useful rules like this with royal pair and bishop "dance"
. Indeed, for a harmonious proportion several rules must be implemented, not just one. Others very useful general rules that I realy like it for a long time ago, which have been already exposed here even by others, are:
-for those who like it more air space-four pawns must fit about on a square.
- or for those who like it more tighter space- two pawns must fit about on the diagonal of the square
-King base diameter must be about 75%-80% from the square.
-King hight must be slightly smaller than two squares
-There may be other useful rules that do not contradict the above rules
If all this general rules match simultaneously with chess pices and the board, it will be a good proportion chess set and a pleasant play game.


very funny video
https://www.youtube.com/watch?v=zJeVid62NZo
I remember watching that a few years back. The test is only a go-nogo test. In the video, the bishop easily slides between the king and queen on a 55mm board, but it will also pass the same test on any board larger than 55mm as well. So which board is the right one? And what about sizing the remaining pieces (knight, rook, and pawns)? Shouldn't they be considered as well?