Do not force your writing into any particular style because it appeals to you, or because you have seen it used by another writer, or because someone else recommends it or says good letterwriting must be so. Style in writing is a very individual matter, and your own writing style will develop and become equally individual the more you write. Try to keep your writing as neat and as clear as possible. A letter written on one sheet, even if it is both sides, in large handwriting is more akin to a note. I recommend you try to write at least four sides for each letter that you send out, and always avoid writing a quick note saying that you will be writing soon.
Some people dislike writing on both sides of the paper.
Writing four sides on only one side of each sheet will put your postage costs up into the next bracket. I generally write a minimum of four sides per letter on two sheets of paper, which comes exactly onto the top end of the lowest postage charge. Finally we come down to the end of the middle part of your letter, where everything is drawn into a close, all the loose ends are carefully tied up and all the news, discussion, comments and arguments have been exercised to the full. The last two paragraphs of this section can be given over to general questions on new subjects, or to last minute thoughts stemming from what you have already written. The last two paragraphs can also be used for general compliments to the other person, stopping well short of flattering them, and perhaps a few passing comments on what you have planned once the letter is safely deposited in the post office.