Both Nikon D5600 and Nikon D5500 have 24.0 MP resolution sensors but Nikon D5600's sensor is APS-C (23.5 x 15.6 mm ) and Nikon D5500's sensor is APS-C (23.5 x 15.6 mm ).
Since Nikon D5500's has a larger sensor area with the same resolution, this means that it also has a larger pixel area hence better light collecting capacity for a given aperture compared to Nikon D5600.
Another similarity between these two cameras is that both Nikon D5500 and Nikon D5600 sensors lack anti-alias (Low-Pass) filters. Removing anti-alias filter increases the sharpness and level of detail but at the same time, it increases the chance of moire occurring in certain scenes.
Below you can see the D5600 and D5500 sensor size comparison.
Nikon D5600 and Nikon D5500 sensors have exactly the same size so none of them has any advantage over other in this area.