Start off slow. Like this. Short sentences or fragments at the beginning. That would be the first paragraph.
Gradually gather steam, constructing sentence of greater length and sophistication. With the occasional short, fragmentary interruption. A few paragraphs of medium-length phrases and clauses, until you fall in to an easier rhythm. It is really rather easy to write in longer units of breath; simple parallelisms (or parenthetical remarks) will suffice. There is nothing particularly difficult about medium, or even long, chunks of discourse.
At this point you should have the confidence that arises from having gone this far without stumbling, even if at the beginning you sorely felt that lack of a firm and steady footing as you began to make your way. At any point you can dial it back and revert to short or medium length sentences. No shame in that. The point is to know that you possess a certain degree of fluency (or fluidity [from the same Latin root: to flow]) as you wend your way up and down the mountain of prose. Later I'll be giving instructions on how to end. At the point you can relax a little more and not even worry about the length of sentences. Your job will be complete.
Right now, though, I recommend that you develop even more elaborate ways of creating complex, but easy-to-follow, units of prose, so that your reader will, also, have the confidence to trust you as a writer, to know that you have everything under control and you move through the middle part of your essay and the average length of your thoughts increases considerably. The ideal to strive for is not length for its own sake, but rather the easy alternation of varying lengths. If you want, you can begin the paragraph with a really long or really short sentence, but then "go smaller" or "bigger" in what follows. The reader should never feel that you are stuck in one particular pattern, or that you are simply unconscious of the desired effect. Instead, he or she should feel that you never bog down into brief, choppy units of sense, or lose him or her in a sea of clauses without end.
Visually, you will be able to discern how long your paragraphs are. Some recommend paragraphs of about six sentences, but there is no hard and fast rule. Once I scoffed at the idea of six sentences, but then discovered that I was following this pattern without even knowing it. I believe my sentences average twenty-five words, though I have not verified this. A good sentence will normally consist of two to three clauses, or a single clause of somewhat greater length. Again, it bears repeating that it is not difficult to create a sentence that goes on and on but still does not leave the poor reader behind.
Beginnings and endings are the hard part. The middle is pretty much a process of going on, and starting and stopping. Between beginning and ending, I would say that beginning is far more challenging. If you can begin well, everything else will typically fall into place. The conclusion should not be a mere repetition of the introduction, but rather a widening gyre of implications and future possibilities. You can wax poetic at that point. As I mentioned earlier, at a certain moment you will stop worrying about how long or short your sentences are, and simply be in a good "groove" or rhythm as you compose your prose. You've already demonstrated that you can write sentences in any length you want, starting or stopping on a dime. You have little left to prove. Just "be yourself" in your writing style. You don't have to stretch yourself any longer, simply to prove that you can. The metaphor of "winding down' is apt for this occasion, although it is always good to reserve a little extra in case you need it for the home stretch.
If sentences have twenty five words, and paragraphs six sentences, then a paragraph will contain one hundred fifty words or so. I realize that what I've written here exhibits some stylistic flaws, perhaps in the use of too many adverbs or "hedges," too many words in air or scare quotes, too many clauses that fulfill no other function than filling space. Still, I feel I have produced another stylistic exercise that allows me to grow as a writer. The total effect of these exercises will only be evident at a later date.