If you explain a no for three paragraphs, you’re asking permission, not setting a limit.

Length signals that you are seeking approval. Three paragraphs of reasons tells the other person the decision is still open and that enough counter-arguments might carry it, because nobody justifies that hard about something settled. The explaining also tends to be for your own comfort rather than their understanding. Short answers feel harsh and are usually received as clear.

Take the next no you have to give and cut it to two sentences, one of which is the answer. Delete the part where you prove it. You can be kind in the tone without adding material.