Pentax 645Z has a 51.0MP Medium format (44 x 33 mm ) sized CMOS sensor and features PRIME III processor. You can shoot at maximum resolution of 8256 x 6192 pixels with aspect ratios of and 4:3. 645Z has a native ISO range of 100 - 204800 and it can save files in RAW format which gives you a wider room for post processing.
Pentax 645Z is not the highest resolution Medium format camera. Fujifilm GFX 100 with its 102.0MP sensor is leading in this class. Check the comparison of Pentax 645Z vs Fujifilm GFX 100 or take a look at Highest resolution DSLR cameras list.
Pentax 645Z DxOMark Sensor Scores
DxOMark is a benchmark that scientifically assesses image quality of camera sensors. Pentax 645Z sensor has been tested by DxO Mark and got an overall score of 101 for its image quality. You can find the details of their analysis of Pentax 645Z here.
Another important feature about Pentax 645Z'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 Pentax 645Z's Medium format sensor compares with other standard sensor sizes.