One of the main questions we get asked in our shop is "how much sugar paste do I need for my cake ?"
The answer of course largely depends on how thick you roll your icing.
We usually recommend rolling out your icing to a thickness of about 5mm, this is because at this thickness the icing should be fairly easy to handle ( without tearing ).
This is also a good thickness to cover any unevenness in your cake after crumb coating it with butter cream
To help you we do have PME marzipan spacers in our shop , which you can use to help you get a consistent thickness whilst rolling out your sugarpaste - go have a look .