What the specification actually requires
The Indian passport photograph is 51×51 mm — square, not the 35×45 mm rectangle used for many visas and most European documents. At 300 DPI that is about 602×602 pixels, and portals typically want the file between 20 KB and 300 KB.
The composition matters as much as the dimensions. The face should occupy roughly 70–80% of the frame, with a plain white or light background, both ears visible where possible, a neutral expression and eyes open and looking at the camera. Shadows on the face or the background are a common rejection reason and are a lighting problem rather than a cropping one.