Is reading what a girl writes in a forum about the details of her body, after knowing her identity, considered a sin? What is the ruling on the girl's action itself? And is the sin of reading these details more severe than the sin of the previous conversation with her?

Your path is full of temptations and violations of Islamic law. If you wish to marry another, take the righteous path by seeking a virtuous wife. Your reading and writing of bodily details are forbidden, based on the Prophet's (PBUH) saying: "A woman should not describe another woman to her husband as if he were looking at her." Allah has warned us against the footsteps of Satan because he does not lead directly to the forbidden, but rather step by step. He has forbidden everything that leads to adultery. Utilize your free time in obedience to Allah, such as memorizing the Quran and studying the Sunnah, for "Two blessings are underestimated by many people: health and free time."