<DockPanel LastChildFill="True" >
            <GroupBox Header="MyTitle"  >
            </GroupBox>
 </DockPanel>

<DockPanel LastChildFill="True" >

<DockPanel LastChildFill="True" >
                <GroupBox Header="MyTitle"  >
                        <DockPanel LastChildFill="True" >
                        </DockPanel>
                </GroupBox>
     </DockPanel>