I know this is a fairly old question, nonetheless I had the same question today and couldn't find any "good" and current numbers, so I'm giving out stats from one of the top 100 sites in Germany. The stats are based on the raw numbers from GoogleAnalytics and have been compiled from 264,069,927 page-views but be aware that the audience of that site is dominantly coming from European countries (while it does have also quite some visitors from all over the world). It's a work-related site (no gaming, no news)

I'll state the browser window resolution and the display resolution on the same site as reference.

First, the width:

• at least 3840px wide: 0.00% (Display: 0.05%)
• at least 3820px wide: 0.00% (Display: 0.05%)
• at least 2560px wide: 0.31% (Display: 2.04%)
• at least 2540px wide: 0.47% (Display: 2.05%)
• at least 1920px wide: 6.64% (Display: 30.17%)
• at least 1900px wide: 19.67% (Display: 30.21%)
• at least 1680px wide: 24.70% (Display: 42.35%)
• at least 1660px wide: 31.16% (Display: 42.78%)
• at least 1600px wide: 32.86% (Display: 48.50%)
• at least 1440px wide: 45.94% (Display: 59.31%)
• at least 1366px wide: 51.16% (Display: 75.09%)
• at least 1360px wide: 51.60% (Display: 75.56%)
• at least 1280px wide: 69.53% (Display: 91.64%)
• at least 1260px wide: 78.60% (Display: 91.68%)
• at least 1152px wide: 84.10% (Display: 93.08%)
• at least 1024px wide: 88.64% (Display: 95.66%)
• at least 1004px wide: 90.96% (Display: 95.72%)
• at least 800px wide: 98.03% (Display: 96.14%)
• at least 780px wide: 98.24% (Display: 96.14%)

And here is the height:

• at least 2160px high: 0.78% (Display: 2.33%)
• at least 1600px high: 32.86% (Display: 48.50%)
• at least 1440px high: 45.94% (Display: 59.31%)
• at least 1200px high: 82.45% (Display: 92.62%)
• at least 1080px high: 87.18% (Display: 94.01%)
• at least 1050px high: 88.05% (Display: 94.22%)
• at least 1024px high: 88.64% (Display: 95.66%)
• at least 900px high: 97.26% (Display: 95.96%)
• at least 864px high: 97.48% (Display: 95.98%)
• at least 800px high: 98.03% (Display: 96.14%)
• at least 768px high: 98.41% (Display: 98.57%)
• at least 600px high: 99.11% (Display: 98.75%)

I took the "Common display resolutions" from https://en.wikipedia.org/wiki/Display_resolution as reference

Two final notes:

• I made an additional entry for each width subtracting 20px because ~20px is often the size of the scrollbar. This explains the sometimes quite big gaps between e.g. 1920px vs 1900px. I did not do the same for heights, but be aware that the "taskbar" (if any) might explain/cause some differences there as well!
• On very small displays, people seem to have windows that are larger than the screen itself, if the data collected by GoogleAnalytics and my calculations are correct.

First, I would go for the W3Counter. At the moment, it shows the statistics based on 75.676 sites which use the W3Counter. For example see http://www.w3counter.com/globalstats.php?year=2014&month=5 for May 2014.

Then, there is the StatCounter. They also collect data of web sites which use their analytics tool and they have a market share about 2% of all web sites. The statistics can be found here http://gs.statcounter.com/.

If you now merge these two statistics, you would get a really credible statistic which will nearly represent the reality (although the statistics have nearly the same values anyway).