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 the original line segment can be calculated by multiplying the length of one piece by 2:
\[ \text{Total length} = 2.9 , \text{cm} \times 2 = 5.8 , \text{cm}. \]
Therefore, Andy's original line segment was 5.8 cm long.