On the one hand there is a simple guideline when selecting passage length to preach – preach a unit of thought. On the other hand, there are many sections that allow for combining units of thought and thereby preaching longer or shorter sections. The epistles, for instance, allow the preacher to combine several paragraphs. Equally, the gospels often allow for the combining of narrative units because that is how the authors often made their overarching points.
I suppose it almost goes without saying, but we can vary length of preaching text.
Sometimes select the smallest possible unit of thought. This might be a couple of sentences that make up a complete thought in the epistles, or a single verse in proverbs, or a very short narrative, etc. If you usually cover more ground it will allow you to give more application, to go deeper in the text, or just to not overwhelm listeners with too much too quickly.
Sometimes combine associated units of thought. This might involve paragraphs that flow together in an epistle or a sequence of narratives in the gospels, or potentially a pair of psalms that go together. Many preachers default to only preaching one unit, or always preaching one chapter, or never really preaching any text, but instead bouncing off one to go everywhere else. Listeners might be very blessed by a combined unit approach.
Sometimes preach whole sections or books. This can easily slip into application-less overview with commentary titles, but it need not be that. The flow of thought in a passage or book is a key discovery. Remember that the epistles were written to be heard in one sitting. Why not give people an overview of the flow of thought, while at the same time being sure to drive home the main point and application of the text. For people used to hearing the same short sections every week, this would be refreshing. It can make a great intro or conclusion to a series, too.
There are other approaches too. How about preaching the same section more than once? How about combining two passages that show the progress among the recipients – such as tracing the story of the church at Ephesus from Acts to Ephesians to Revelation chapter two?
Simple point about preaching text length . . . vary it.