I would suggest you do an experiment in a sandbox. I doubt very much you will find any such sizing guidelines except from possibly SAP directly.
So configure your sandbox for the basic QM stuff.
Add in say 5% of your data that you expect to load.
Take sizing readings directly before and after your loads.
Figure out the difference and multiple by 20, (if you load 5%. if you load 1% multiple by 100, etc..).
Craig