Nikon D3300 has a 24.0MP APS-C (23.5 x 15.6 mm ) sized CMOS sensor and features Expeed 4 processor. You can shoot at maximum resolution of 6000 x 4000 pixels with aspect ratios of and 3:2. D3300 has a native ISO range of 100 - 12800 which can be boosted to 25600 and it can save files in RAW format which gives you a wider room for post processing.
Nikon D3300 is not the highest resolution APS-C camera. Canon M6 MII with its 33.0MP sensor is leading in this class. Check the comparison of Nikon D3300 vs Canon M6 MII or take a look at Highest resolution DSLR cameras list.
Nikon D3300 DxOMark Sensor Scores
DxOMark is a benchmark that scientifically assesses image quality of camera sensors. Nikon D3300 sensor has been tested by DxO Mark and got an overall score of 82 for its image quality. You can find the details of their analysis of Nikon D3300 here.
Another important feature about Nikon D3300'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 Nikon D3300's APS-C sensor compares with other standard sensor sizes.