The longest ayah (verse) in the Qur'an is found in Surah Al-Baqarah. It is verse 282, commonly known as Ayat ad-Dayn (the Verse of Debt/Financial Transaction). This single verse spans approximately one full page of the standard Mushaf and deals comprehensively with the rules governing financial transactions, debts, and contracts.
Ayah 282 of Surah Al-Baqarah instructs believers to write down their debts and financial obligations, to have witnesses present, and provides detailed guidance on the proper way to document such agreements. It covers the appointment of a scribe, the number and type of witnesses required, and the circumstances under which documentation may be exempted (such as immediate hand-to-hand trade).
Surah Al-Baqarah is also the longest surah in the Qur'an, with 286 verses. It contains several other notable passages including Ayatul Kursi (verse 255) and the final two verses that are recommended for recitation before sleep.
The other surahs listed - Al-Tawbah (Chapter 9), Al-Imran (Chapter 3), and Al-Nisa (Chapter 4) - all contain lengthy verses, but none surpasses verse 282 of Al-Baqarah in length.