So what is balance?
Simply, it's the attention a writer gives their material. And yes, I'm being intentionally vague when I say “their material”.
Let's consider it in the context of a 5 Paragraph Essay, about 1000 words long, no tangents, and each paragraph addresses its own specific, related topic. The expectation is each paragraph averages about 200 words. But let's say this essay's paragraphs are, respectively, 200, 400, 150, 100, and 150 words. The enormity of the second paragraph in comparison to those that follow makes it stand out. If it's a decent paragraph, it will identify its topic and then explore it in detail. This is all good, but the other paragraphs being a about a third or fourth in length will pale in comparison. They may be well written paragraphs, but their brevity suggests a few things. The author cared much more about one topic than the others, and these other topics were not addressed to the same extent. This can weaken the overall essay, no matter how good the longer paragraph was.
The solution may or may not be to simply make the other paragraphs three or four times as long: the issue would be if the author can or needs to make these paragraphs as detailed as the first one. If they find the first topic much more interesting than the rest, and the others are there to fill space, then they may be able to produce more words, but the writing won't be as strong. The solution may be scrap the other paragraphs and break the long one into more specific topics.
A great example of this is on annotated bibliographies. I will see a long, almost lyrical summary of a source followed by a few haphazard sentences of evaluation. This is not a reflection on the source but on the author's attention to the sources: the better summary shows the author paid careful attention to one source, but not to the rest of their research.
Issues of Balance are more prevalent in heavily structured forms, like annotated bibliographies and rogerian arguments (for example: a few sentences are spent on the opposition and a few paragraphs on the author's stance), so I try to emphasize it more when teaching these forms. It's easier to point to the length or number of paragraphs and show how and why the longer and shorter ones, generally, have more detail and show the author was more careful and attentive to one more than another.
However, length is only a superficial indication. Some paragraphs need more detail and attention and will therefore be longer. Others may just need a few sentences to get their job done. The real issue is how the paragraph is written, which is why I used the 5 paragraph essay in my earlier example: each body paragraph has basically the same purpose, so each should have about the same amount of detail and attention, which will likely reflect in their all being close enough in length that none appears significantly longer or shorter than another. You need to read your own writing purposefully: what purpose does each section and paragraph have does it have the detail needed to meet that purpose?
Looking beyond the word count and thinking about purpose and function is important, and I think that focusing on Balance is a good way to teach and emphasize this kind of reading. It may be hard to quantify, require a fair amount of specification, and vary from situation to situation, making it hard to teach, but that’s no reason not to. The overall lesson is to ask yourself what the purpose of the paragraph is and if it is getting the attention and detail it deserves.