Currently this feature is not supported. Using the Graph item you will produce a single pie chart. Thus our suggestion is to analyze the data before passing it to the report, add Graph item
for each pie chart you want to render and set appropriate size for each of the items.
If you have any further questions, please elaborate on the scenario as what is the expected report layout, what is the used data structure and how it is retrieved.
Have you tried the new visualization options in Telerik Reporting Q2 2013? You can get them from your account.