I have a 21 inch monitor with a resolution of 1680x1050, my sister has a laptop with a 15 to 17 inch monitor with the same resolution, most people I know don't have a resolution under 1280x800 (or similar)
Steam survey has the following information:
http://store.steampowered.com/hwsurveyPrimary Display Resolution
1024 x 7685 .12%-0.17%
1152 x 864 1.04% -0.02%
1280 x 720 0.69% +0.03%
1280 x 800 5.28% +0.20%
1280 x 960 0.95% -0.02%
1280 x 1024 11.80% -0.47%
1360 x 768 1.42% +0.03%
1366 x 768 6.50% +0.25%
1440 x 900 9.20% -0.21%
1600 x 900 4.05% +0.17%
1600 x 1200 0.81% -0.07%
1680 x 1050 18.01% -0.23%
1920 x 1080 21.78% +0.72%
1920 x 1200 7.80% -0.11%
2560 x 1440 0.65% -0.03%
Other 4.92% +0.62%