If the line segment bisector divided Andy's original line segment into two equal pieces that are each 2.9 centimeters long, then the original line segment must be the sum of the lengths of these two pieces.
\[ \text{Original length} = 2.9 , \text{cm} + 2.9 , \text{cm} = 5.8 , \text{cm} \]
Thus, Andy’s original line segment was 5.8 cm long.