From Mechanics in Guide part of ABC of Plain Words by Sir E Gowers (1951)

Letters, reports, memoranda and other documents would be unreadable if they were not divided into paragraphs, and much has been written on the art of paragraphing. But little of it helps the ordinary writer; the subject does not admit of precise guidance. The chief thing to remember is that, although paragraphing loses all point if the paragraphs are excessively long, the paragraph is essentially a unit of thought, not of length. Every paragraph must be homogeneous in subject-matter, and sequential in treatment of it. If a single sequence of treatment of a single subject goes on so long as to make an unreasonably long paragraph, it may be divided into more than one. But you must not do the opposite, and combine into a single paragraph passages that have not this sort of unity, even though each by itself may be below the average length of a paragraph.