Ohms per square is a dimensionless square area of resistive material, the length and width of the resistor being of equal size, having an Ohmic value equal to the sheet resistivity of the resistive material deposited onto the copper surface. A 25 ohm sheet resistivity material would yield a 25 ohm resistor whether 1 mil wide by 1 mil long, or 1 inch wide by 1 inch long.