A common problem with essays by beginning writers is that the paragraphs are too short. This fault is especially common in handwritten essays, where the small number of words per line makes the paragraph appear longer than it is. Having no real idea of what a paragraph should do, writers put in breaks whenever they feel the essay needs one visually.
Paragraphs do perform a visual function, but they are more than just dams in a flood of writing.
The paragraph organizes both visually and thematically. Manuals like this one leans toward short paragraphs for the same reason. Essay format, however, allows for some longer, more detailed paragraphs. Usually a paragraph that is becoming too long visually has also become too long thematically, incorporating more than one main idea. It is often stylistically effective to vary the length of your paragraphs, just as it is to vary the length of your sentences.