It is all just a matter of preference. I've seen numbers as low as 1.5 fold increase and as high as 2.5 fold as cited in the article. I normally shoot for a 2 fold increase in size. Remember, you will not be fermenting the dough in a graduated glass beaker, as they were in the article, but rather you will be "eyeballing" the dough to ascertain the magnitude of size increase. Why do I use a 2 fold increase? Because it is easier to say "Yup, that looks to be about twice as large as it was when I started".
Tom Lehmann/The Dough Doctor