Generally, the King piece base diameter of a Staunton design chess piece should be about 3/4 of the chessboard’s square size. Yes, it’s really that simple!
For non-regulation chess play you can of course deviate from this as you wish, yet the guideline is useful. A smaller square size relative to the pieces will create a denser or crowded feel, while a larger square size will create the opposite effect – a more sparse and open playing field.
For personal use, these are guidelines. Officially however, the U.S. Chess Federation’s Rules of Chess, 5th edition, says regarding board size, 41C page 227:
“The guideline for determining the proper square size for a Staunton chess set is that the King should occupy around 78% of the square.(Dividing the base diameter of the King by 0.78 will yield the proper square size). An acceptable square size may be up to 1/8 inch larger than this number, but not smaller.”
Example 1: For a chessboard with 2″ Squares, the King’s base should be about 1 1/2″ (about 75% of 2″).
Example 2: For a 1.75″ King base, divide by .78 which is approximately 2.25″, the recommended square size.
Let’s look at a series of visual examples.
(67%) Regulation Chess Set with 1 1/2” King base and Vinyl board 20” Square and 2 1/4” squares.
(84%) Club Chess Pieces with 1 1/2″ King base and 17” Vinyl board with 1 7/8” square. The same club pieces can be played on a smaller playing surface.
(73%) Timeless pieces with 1 7/16″ King base on 18” US Made wood chess board with 1 15/16” squares.
(72%) Timeless pieces with 1 7/16″ King base on 21” USA Solid Hardwood Walnut and Maple Chess board with 2” squares
For unique or theme chess pieces other than Staunton style, such as a civil war chess set, these are still good guidelines, but the piece shape can affect the ideal size. A full body horse with a mounted solider may reach beyond the square.
In this example, the king’s base size is actually 58% of the square.
(58%) Civil War Chess Set with 1” King base and 1 5/8” Square. The knight piece balances extra square size.