The denouement answers the smaller questions the climax left open and, more importantly, shows the changed state. Without it an audience knows what happened and not what it meant. With too much of it, the piece explains its own ending, which is the fastest way to make a good climax feel smaller in retrospect.
A useful rule of proportion: the denouement should be shorter than the audience expects, and should show rather than state. One image of the changed world beats a paragraph of narration about it, every time.
In commercial work the denouement is where the call to action usually sits, and the same rule applies. An ask that arrives after the piece has finished settling reads as a separate advert stapled on.
What is a denouement?
The passage after the climax in which consequences settle and the new state of things is shown. It answers what it meant, not what happened.
How long should a denouement be?
Short. Long enough to show the changed state, and no longer. If it is explaining the ending rather than showing its consequences, it is too long.