Catechism of the Catholic Church §1460 — excerpt
The penance the confessor imposes must take into account the penitent's personal situation and must seek his spiritual good. It must correspond as far as possible with the gravity and nature of the sins committed.
This translation is under copyright, so only the cited words are quoted here. Follow the link for the full text. Translation: English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va.
Also cites: Council of Trent (1551): DS 1690 · Council of Trent (1551): DS 1691