Below you can see the front-view size comparison of the Nikon D5300 and Olympus E-M5 II. The Olympus E-M5 II is clearly the smaller of the two cameras. Its body is 1 mm narrower, 13 mm shorter and 31 mm thinner than the Nikon D5300.
The chart below depicts how two cameras compare with average dimensions of the cameras in their class.