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Here's an approach that might work for you:

Select the incorrectly sized line. Duplicate it. Resize the new line to the length you want. Align it parallel to the existing line, in the position you want it, just displaced a cm or so toward the appropriate side of the canvas. Now one by one go through the lines connected to the old line, selecting each one in turn, then dragging its connection point from the old line to the new. When you've done them all, delete the old line, and shift the new one into place.