Given length of a side x...
Tip to tip across the hex is 2x.
Height of the hex flat side to flat side is 2x(sqrt(3/4)) or about 1.732x.
Area of the hex is 1.5(x^2 (sqrt(3)) or about 2.56x^2.
Example: Making graph paper with 4 hexes per square inch
Hexagon with a side length of x... The area of that hex would be about...
So for 4 hexes per square inch...
4 * 2.6 (x^2) = 1
x^2 = 1/10.4
x^2 = .096
x = .31 inches per side.