If the wire has a circular cross-section - the usual case - use the formula for the circle: pi x radius squared.
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If the diameter doubles (x2), the cross-sectional area quadruples (x4).
Imagine the wire is straight, now cut through at right angle to the centre line, the exposed surface is the cross sectional area, on a round wire it = pi * radius2 (area of a circle)
Other things being equal, more cross-sectional area will cause less resistance.
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It quadruples.