The exact stacking space required will vary greatly with material thickness and flexibility, so it’s best to consult an expert. However, as a general rule, a light, unlined curtain will require 10 to 15 percent of the panel width to stack, while a heavier, lined drape will need 20 to 25 percent.
Therefore, if you want your drape to open wide enough to reveal the whole window, the rod or track should be this percentage wider than the window itself.