I have a cuboid with height, width and length not of same scale. When I Smart Project UV map for this, I get islands of same size.
I think I must get islands of different sizes as height, width and length are not of same scale for my cuboid.
Am I doing anything wrong or is it normal ?
It sounds like you have scaled your mesh. Unwrap operations should always be done with the object scale set to 1 (to avoid problems like yours).
In object mode if you select the object and look at the scale in the properties region, you will see that it is not all 1.
Press CtrlA > Scale, to apply the scale.
Then in edit mode re-unwrap the mesh, and it will have the proper scale.
As an aside, if you were to use the Unwrap operation, blender would warn you with this message.
Object has non-uniform scale, unwrap will operate on a non-scaled version of the mesh.
