If the line segment bisector divides the original line segment into two equal pieces that are each 2.9 centimeters long, then the total length of Andy's original line segment is the sum of these two pieces.
To find the 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.