Take a look at the example at "All features\Welcome !\Chart styles\Standard\Pie\Multiple Pies" in the features demo shipped with TeeChart installation (or download the precompiled demo here)
In it, you'll see how you can define the rectangle to be used by a series, at the series' BeforeDrawValues
event. Ie:
procedure TPieMultiple.Series1BeforeDrawValues(Sender: TObject);
begin
Chart1.ChartRect:=Rect(10,10,Chart1.Width div 2, Chart1.Height div 2);
end;