Below you can see the front view size comparison of Canon 1200D and Nikon D3300. Nikon D3300 is clearly the smaller of the two cameras. Its body is 6mm narrower, 2mm shorter and 2mm thinner than Canon 1200D.
Canon 1200D vs Nikon D3300 - Front View
Canon 1200D vs Nikon D3300 - Top View
The chart below depicts how two cameras compare with average dimensions of the cameras in their class.
Camera Size Comparison
Select Two Cameras for Size Comparison