You can change the dimensions and elevation of all objects in Sweet Home 3D with the exception of walls and rooms that have a few constraints.

For your model primitives:
• Use the Generate 3D Shape plugin to create any 8-point form you want
• Use the Basic Shape Generator plugin to create basic forms
• Download and import the DodecagonShapes library from dodecagon.nl
• Combine shapes and export/import to get a new shape

Some useful manuals, for example how to change material names, you can find on the Dodecagon Manuals page. And of course there are the Sweet Home 3D users guide and the Sweet Home 3D Tips and Tricks page