If Lucia constructed line segment \( \overline{CD} \) as a copy of line segment \( \overline{AB} \), then it means that the lengths of \( \overline{AB} \) and \( \overline{CD} \) are equal. Therefore, the correct statement is:
\( AB = CD \).
This is true because a "copy" in geometric construction using a straightedge and compass means that the two segments have the same length. Hence, the answer is:
AB = CD.