Was Sai Baba's existence limited to his physical form? What does the provided text say about his essence?

No, the text strongly refutes the idea that Baba's existence was limited to his physical body. Chapter 43 states that it is improper to think of him as fitting only within a body of three and a half cubits or belonging to a specific caste. He is described as the personification of Brahman, a powerful being of miraculous avatars who fills the moving and non-moving world. After leaving his body, he merged into the unmanifest and his true form, remaining firmly imprinted in the hearts of his devotees. His essence is portrayed as unbroken and all-pervading consciousness.