If you're thinking of a paragraph as a unit of text:
* Sentence: A single complete thought.
* Word: The smallest unit of meaning.
* Chapter: A larger section of a book.
* Section: A distinct part of something larger, like a document.
If you're thinking of a paragraph as a visual element:
* Blank space: The absence of text.
* Image: A visual representation.
* Table: A structured arrangement of data.
If you're thinking of a paragraph as a structural element:
* Line: A single line of text.
* Verse: A line or group of lines in a poem.
* Stanza: A group of lines in a poem.
Ultimately, the best antonym depends on what aspect of "paragraph" you want to contrast.