Canon S120 has a 12.0MP 1/1.7" (7.44 x 5.58 mm ) sized BSI-CMOS sensor and features Digic 6 processor. You can shoot at maximum resolution of 4000 x 3000 pixels with aspect ratios of 1:1, 5:4, 4:3, 3:2 and 16:9. S120 has a native ISO range of 80 - 12800 and it can save files in RAW format which gives you a wider room for post processing.
Canon S120 is not the highest resolution Small Sensor camera. Kodak Astro Zoom AZ651 with its 21.0MP sensor is leading in this class. Check the comparison of Canon S120 vs Kodak Astro Zoom AZ651 or take a look at Highest resolution Compact cameras list.
Canon S120 DxOMark Sensor Scores
DxOMark is a benchmark that scientifically assesses image quality of camera sensors. Canon S120 sensor has been tested by DxO Mark and got an overall score of 56 for its image quality. You can find the details of their analysis of Canon S120 here.
Let's look at how the size of the Canon S120's 1/1.7" sensor compares with other standard sensor sizes.