How long should a sentence be? Six months? Ten years? Fifteen to life?
So what's the crime? Trying to say that, in certain situations, there is a "correct" number of words for the space between the opening capital letter and full stop of the grammatical unit we call a sentence. And believe me, there are those who would have you think that certain types of writing (let's say website copy) benefit from "sentences of about 20 words."
There is only one thing to remember when it comes to sentence length: variety.
It would not surprise me if it emerged that the CIA has an advanced form of torture involving the reading out of hundreds of sentences that are all the same length and rhythm. Think you could take it? Here's a test.
"I am going to tell you how to be successful in your business. It is a foolproof method that anyone can follow. There's no catch and nothing to pay upfront. In the pages that follow I'm going to explain how it works. You have to read every sentence for the system to succeed. There are about 500 of these sentences and they are all like this. Lose interest once and you will definitely fail. ..."
OK, OK, I'll tell you anything you want to know! Life is just too short to have to put up with that. Much better to keep your reader interested by simply varying the number of words you use. Shorter sentences can be really effective. Longer sentences are useful for developing ideas, building lists or to highlight what follows. Like this. Easy.
However you do it, vary your sentence length to keep your reader active. It'll be good for both of you.