The provided Tafsir Ibn Kathir does not address the calligraphic lengthening of the letter 'Ba' (ب) in "Bismillah" (بِسْمِ). The commentary focuses on the meaning, virtues, and names of Surah Al-Fatihah, as well as the significance of "Bismillah" as an ayah and its role in prayer.

Linguistically, the Arabic script allows for certain elongations (مدّ) for aesthetic or emphasis purposes in calligraphy, but this is a calligraphic convention rather than a grammatical or lexical feature that alters the meaning or pronunciation. The 'Ba' in "Bismillah" is a preposition (حرف جر) meaning "in" or "by." Its form is standard in written Arabic, and any lengthening in specific manuscripts would be an artistic choice.

The tafsir sources emphasize the importance of the phrase "Bismillah" itself, noting its inclusion as an ayah by many scholars and its role in beginning Surah Al-Fatihah, which is called "Umm Al-Kitab" (Mother of the Book) because the Qur'an starts with it (Source 2). However, they do not discuss the visual presentation of individual letters.

📚 Sources: Classical tafsir (multi-source)