The values for Size X (width), Size Y (height) and Size Z (depth) define the size of the object.
These values control the cube’s number of subdivisions in the X, Y and Z directions. This is then important when editing a given primitive with various tools (see Mesh menu. The more detailed the subdivision the smoother any deformations will appear. Boolean operations in particular require a higher degree of subdivisions (only where necessary, of course) for better results.
If this option is enabled and you make the cube editable, a separate object will be created for each side of the cube. This can be useful if you want to edit each side separately. You can only select this option if Fillet is disabled.
To fillet (i.e. round) the object’s edges, enable the Fillet option. Fillet Radius is the radius of the curvature, while Fillet Subdivision defines how fine the fillet appears. The more segments you specify, the smoother the fillet. To create a chamfer instead of a rounded fillet, set Fillet Segment to 1.
The radius of the fillet can be no larger than half of the value of the smallest of the cuboid’s dimensions. If your cuboid has, say, a width of 100 units and a height and depth of 300 units, the maximum fillet radius is 50 units