Document would be extremely difficult to read. Where you break your entry should. Therefore, be a matter of common sense.
Generally speaking, a paragraph should not be more then about fifteen or twenty lines and, in most cases, much shorter than. There are no fixed rules, but bear in mind that the point of a paragraph is clarity of expression and that short paragraphs are infinitely preferable to a long ones.
Business letters tend to be’ aligned left’ which means that all paragraphs star on the left-hand side of the page and none are indented. This gives a clean, neat look and appears t liker trade. When preparing semi-professional letter-to be school head teacher for example-it may be appropriate to indent the first line of each paragraph since this is a throwback to handwritten letter style and might seem more elegant.
Handwritten unready almost always indent at the beginning of each paragraph on the basis that anything that makes handwriting easier to read is a good thing.
The rest of the layout will depend on the length of your letter. Most of us have received letters which begin at the top corner of the page and then fill the entire bland space with no margins and hardly any paragraphs. This is not going to facilitate the reading of your letter.