The cross-sectional area of a wire is the size of the face of the wire if it was cut vertically perpendicular to it's length. The cross-sectional area is independent of wire length and is typically uniform across the entire length.
Because a wire is essentially a cylinder and the cross-section of a cylinder is a circle, the cross-sectional area of a wire can easily be calculated using the equation for the area of a circle. Multiplying pi (3.14) by the radius squared results in the area of the wire's cross-section. The radius can be determined by measuring the diameter of the wire and dividing it by two.