Assuming that one cell is 6" wide; standard board lengths are 6, 8, 10, and 12. For example a 4'x4' board is 8x8 and a 3'x6' board is 6x12.
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Cell size needs to be a multiple of sub-grid in order for both to properly align
Show sub-grid
Sub-grid needs to be a common factor of the Cell size or it will not align
Assuming that one cell is 6" wide at 25MM; use 2