I've always dismissed efforts to size the 'content marketing market'. The definition of content marketing's very squirrelly, meaning:
a) if you size it including A, B, and C, then people will invariably say it should also include D, E, and F (but not B, or C), and
b) if you ask marketers to give you a number, then you're at the mercy of however each marketer defines it (combining apples and oranges).
Last year, when setting up our big annual survey of marketing leaders, we sidestepped the definition mine-fields by asking marketers to tell us how much of their budgets would go to content creation. Taking that data and applying some approximations (% of revenue going to marketing budget and total domestic industrial revenues) gets you to my headline figure: $10 billion spent on content (all of it, not just 'pull content', 'media content', or whatever content ghetto you want to define) in the US in 2016. Considering US GDP is about one-quarter of global GDP, that brings us to a ballpark figure of $40 billion annually in global marketing content spend in 2016.
The figure's admittedly a ballpark estimate, but at least it describes clearly what it contains (marketing content), and it's pleasingly well lower than other, more hyperbolic estimates for content marketing that I've seen bandied about.
For those with Forrester access who want to dive a bit deeper into spending figures, see the full content spending report.
For those without it, a couple more interesting findings from recent content data:
Marketers and their agencies disagree subtly on how well brands do content
Across a broad selection of marketing objectives, content marketing is a top five priority
All agencies expect more content dollars, but digital agencies are growing fastest