Fujifilm X100 has a 12.0MP APS-C (23.6 x 15.8 mm ) sized CMOS sensor and features EXR processor. You can shoot at maximum resolution of 4288 x 2848 pixels with aspect ratios of 3:2 and 16:9. X100 has a native ISO range of 200 - 12800 and it can save files in RAW format which gives you a wider room for post processing.
Fujifilm X100 is not the highest resolution APS-C camera. Fujifilm X-H2 with its 40.2MP sensor is leading in this class. Check the comparison of Fujifilm X100 vs Fujifilm X-H2 or take a look at Highest resolution Compact cameras list.
Fujifilm X100 DxOMark Sensor Scores
DxOMark is a benchmark that scientifically assesses image quality of camera sensors. Fujifilm X100 sensor has been tested by DxO Mark and got an overall score of 73 for its image quality. You can find the details of their analysis of Fujifilm X100 here.
Anti-Aliasing Filter
Another important feature about Fujifilm X100's sensor is the lack of anti-alias (Low-pass) filter. Removing anti-aliasing filter increases the sharpness and level of detail but on the other side it also increases the chance of moire occurring in certain scenes.
Let's look at how the size of the Fujifilm X100's APS-C sensor compares with other standard sensor sizes.
Comparison image of Fujifilm X100 Sensor Size with other standard sensors