Once you have tiled surfaces such as floor, walls or objects, you can add additional (rectangular) layers that can be tiled themselves too.
Click over the surface in which you want to create the new tiles layer, select command Add tiling area:
Specify the width and height of the layer:
Click OK, then move the layer in the scene (in the next image underneath a sanitaryware):
Finally, get to the Components window in the left pane, select the item you want to tile the new layer with, drag and drop it over the layer. In the next image you can see an orange 10x10 cm tile:
What about if you need to change the dimension of this layer? Click over it, and you will see this group of commands:
Note that the Properties window on the right refers to the pattern, and allows you to change orientation, grout value and color, alignment.... If you want to open the Properties window related to the tiles layer structure, select command Change selection, Tiling area. Now you have available the controls that allow you to change width and height.