Here is a sentence from a well-known novel:

He walked up hill in the mire by the side of the mail, as the rest of the passengers did; not because they had the least relish for walking exercise, under the circumstances, but because the hill, and the harness, and the mud, and the mail, were all so heavy, that the horses had three times already come to a stop, besides once drawing the coach across the road, with the mutinous intent of taking it back to Blackheath.

http://www.victorianlondon.org/books/tale-02.htm

At 79 words and more than four lines, even with a semi colon break after the first 20 words, this sentence taxes the reader. We may forgive Charles Dickens long sentences like these in The Tale of Two Cities because literary styles change and Dickens is a great author. However, our readers will not forgive us if our sentences seem interminable.  Readers get lost in long sentences, sometimes because the writers themselves got lost. If we keep our sentences shorter than two lines, we will find it easier to order our thoughts and our readers will find it easier to follow our thinking.  As a rule of thumb, a sentence longer than two lines is too long.

Long paragraphs, like long sentences, can put off our readers.  If our essay has paragraphs that run more than a page or take up most of a page, one look at them may cause our readers, like the horses on Dickens’ hill, to call it quits. A shorter paragraph—perhaps five sentences—is more inviting and apt to be easier to understand.