Writing: A good sentence is a balancing act

Source: old-picture.com

A sentence is a unified thought, but there's no absolute standard of unity.

How much you put into a sentence, how many words, phrases, and clauses used to support the main idea depends upon the circumstances.

This is because a sentence is also a balancing act.  

Minto says, 

In difficult cases you cannot settle the right proportion without going beyond the sentence itself. You must consider the subject on which you are writing, whether it is simple or abstruse; the scale on which you are expounding it, whether your exposition is brief and condensed or open and diffuse; the nature of the attention that you may reasonably expect from your reader, [whether] he reads you running or sits down to study you with deliberate care. On this last point we are particularly apt to deceive ourselves.

There also is no mechanical rule of sentence length.

In fact, there's no mechanical rule of anything. You absolutely should internalize the basics, but once you've got the rules down, you're ready to break them, to make them your own.  You move from following a recipe to becoming, through practice, observation, and experimentation, a great cook.

 Source: Minto.