A web page should be as long as it needs to be to satisfactorily deliver on the promise of its headline or the link that got people there. That’s the definitive answer.
Now, in order to satisfactorily deliver on the answer to the question “How long should a web page be?”, I realise that the statement above is going to need some further explanation, but that proves the point.
What we should avoid, after more than 25 years of search engines, is being a slave to SEO theories. Yes, it’s true that the top-ranking articles about important subjects are more likely to be 2000 words long than 200. But if we can nail something in a few paragraphs or less, we’re more likely to get the appreciation and links that we deserve.
I also think that the point at which an article should finish is the point at which different readers will want to go in different directions. If we’re describing something that has two types, or two functions, and most readers will be interested in one or the other (but not both), we shouldn’t make half of them wade (or skim) through irrelevant stuff to get to the bit they want. End the article, and offer them links to separate articles on the two options.
While we’re still covering material that will be of interest of every reader, keep on going. You’ll easily see how this approach translates even to the simplest things, such as product pages.