For a balloon bouquet with 3 balloons in different heights, similar to our "Simple & Elegant" centerpiece, the ribbons should be 2.5m, 2m and 1.6m long. This gives you enough height for the balloons plus about 50 cm of ribbon on both ends for curling. If you are only going to curl the top end, hanging down from the balloon, you could cut them about 50cm shorter.
But I would advise that you first do one bouquet as a test, to see if the heights suit your design and environment.
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