Here is a PS4 vs. Xbox One console size comparison: (PS4 is on the left, click an image for a larger view.)

Thanks to ThelVluffin for creating the above renders.

Update: Here's another more detailed console size comparison that includes PS3 and Xbox 360. This one also includes the actual dimensions of each system: 

Above image source: PSMania

One more:

Before you go, check out the PS4 vs Xbox One in-depth comparison that includes hardware stats, exclusives and a lot of other interesting pieces of information about the two next-gen consoles.