It all depends on how much drop is needed in the garland. Let's assume that the garland extends from the midpoint of the ceiling to the 4 corners of the room and there is no drop. We have a rectangle 74*42 and the length of the garland is the sum of the lengths of the diagonals which cross at the centre. So each diagonal is sqrt(74^2+42^2)=85.09' approx, so you would need twice this=170.18'. If you are also putting garland along the walls, you need to add in the perimeter: 2(74+42)=232'.
The problem becomes more complicated when you allow the garland to drape. The lengths of the pieces are the lengths of the catenaries (similar to parabolas) formed by the drape, and you would need to state the drop (the greatest distance from the ceiling). You would also need to say whether each garland is attached to the centre of the ceiling making 4 catenaries or whether the garlands drape from corner to corner crossing over at the midpoint.
My guess is that, since you asked this question in May 2012, you will already have solved the problem (which I've only just seen)!