If your playing field is bigger or smaller, adjust accordingly. With a bigger table than 6 ruler lengths, expand the ruler units to match the table size, or tape off the edges of corners of your gaming area to stay within 4 to 6 lengths.
If your area is smaller than 4 rulers, reduce the ruler to 6 units length. Weapon ranges behave accordingly, so direct-fire is now between 1 and 6 units, and artillery starts beyond 6. Spotting behaves just the same.