Below you can see the front view size comparison of Canon T6 and Nikon D3400. Nikon D3400 is clearly the smaller of the two cameras. Its body is 5mm narrower, 3mm shorter and 2mm thinner than Canon T6.
Canon T6 vs Nikon D3400 - Front View
Canon T6 vs Nikon D3400 - 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