Width, Height, Depth

width: Width (in meters) of the sides on the X axis. Default value: 1
height: Height (in meters) of the sides on the Y axis. Default value: 1
depth: Depth (in meters) of the sides on the Z axis. Default value: 1

<!-- Box with Zero position -->
<a-box
   id="box"
   position="0 0 0"
   width="1"
   height="2"
   depth="3"
   color="#4CC3D9"
></a-box>

The different values of x, y and z are clearly visible.