Just because I thought it was interesting, I shared this on Twitter this morning:
95% of traffic to CSS-Tricks has a screen resolution of larger than 1024x768.
And I got all kinds of responses like: That's such a skewed statistic!!!
How are my own stats on my own website skewed?
OK, I know what they meant, they meant that that 95% number isn't indicative of most websites, it's only CSS-Tricks visitors. So people shouldn't quote that number out of context or use it to inform design decisions on other websites.
But that highlights an interesting point. Who cares what the global average of internet user screen size is? The only stats that matter are your own. Unless you are starting a new project from scratch and need a baseline, in which case I think you are better off finding a related website and kindly asking for their numbers than using a global average.
That said, I'm not sure that particular statistic will inform my design decisions either. As I tinker with a redesign, I plan to support all screens large and small.