What we call it, what we say, and how we say it, all depend on your objective.
So does the length. For example, if you want to give Google and other search engines something unique to chew on — positioning
you as an authority in your field and improving your Search Engine Optimization — you can go long ... 2,000 words or more
(if there's that much to say). But also use visuals and time-honored typographic "tools" (such as subheads, crossheads, bold leads,
white space, insets, pull quotes and sidebars) to
make it visually attractive and easy to scan.
On the other hand, to stimulate immediate response and/or reader
comments, keep it short — say, 500-600 words. (If it's much less than that, Google may not consider it unique or substantive.)
Then again, if it's to appear in a paper publication
or on someone else's website, the piece will have to meet their editorial specifications. And if it's a broadcast press release, write so as to
give editors easy editing options.
In any case, if your ultimate goal is to sell, include an offer and call to action, and don't bury that at the bottom.
Another rule of thumb is the advice the Walt Disney's Cheshire Cat gave to Alice: