What are the functions of a body paragraph in the thesis/proof paper?
One way of thinking of them is in terms of the claim-data-warrant model suggested by Stephen Toulmin.
Generally, each main body paragraph in a thesis/proof paper makes a claim in support of the thesis. It is then the writer's role to provide the evidence (or, data) that supports this claim, and then to also provide a "warrant," which explains how the evidence in fact supports the claim.
Let's look at a sample body paragraph written by Lori Fernald. This comes from a paper written on Alice Walker's The Color Purple. In it, Lori argues that the character Shug Avery can be interpreted in terms of the Magician archetype discussed by Carol Pearson in her book The Hero Within. In the sample paragraph, then, Lori claims that Shug is a Magician. To back up this claim, she provides data from The Color Purple. She also establishes her warrant by providing supporting material from a critical article written by Thomas Marvin and by pulling out the implications of both Walker's and Marvin's texts.
Shug can inspire Celie because she has already journeyed and become a Magician. Shug Avery embraces all aspects of life by creating her own way of living based on understanding and holism, including developing her own spirituality which replaces the traditional patriarchal religion. Spiritually, Pearson's Magician "celebrates the experience of God in everyone . . ." (Pearson 23). Similarly, Thomas Marvin asserts that " . . . Shug Avery promises her followers a new relationship between the individual and the world, one based on an understanding of the holiness of all living things and the spiritual power of the spoken word" (411). Shug demonstrates her Magicianhood when she says,
. . . [God] ain't a picture show. It ain't something you can look at apart from anything else, including yourself. I believe that God is everything . . . Everything that is or was or ever will be. And when you can feel that, and be happy to feel that, you've found It (202-3).
Shug's concept of the divine incorporates everything, "keeping alive the vital connections between the past, the present, and the future" (Marvin 419). Shug even views sex as spiritual: "God love all them feelings. That's some of the best stuff God did. And when you know God loves 'em you enjoys 'em a lot more" (203). Shug's all-inclusive attitude incorporates sexuality as well as the physical act of sex. She is a practicing bisexual, embracing all sides. When considering Pearson's Magician, most do not usually think of "embracing all parts" as including sexuality. However, we can see clearly that sexuality relates to Magicianhood as much as anything else. Shug understands male and female sides, and realizes that everyone has both natures in them.
Richard Marius, a historian and writing teaching at Harvard, suggests: "Build paragraphs on a thought you express in the first sentence."
Lori has clearly done that here. She orients the reader at the outset with her claim, and then proceeds to build a meaningful unit of meaning.
Marius also proposes "nearly every sentence in a good essay looks backward and forward." Reread Lori's paragraph with this in mind, noting how each sentence builds on its predecessor and anticipates the one that follows. Such mindfulness is what creates paragraph coherence.
How long should a body paragraph be? There's no set limit. But if you provide sufficient data to support a claim and then work that data the way that Lori has, looking at the implications of individual pieces, then good-sized length should naturally emerge.