From MSDN: Ribbon layout and resizing (about halfway down the page in small text):
Control groups
RibbonControlGroup. RibbonControlSizeDefinition RibbonControlGroup. RibbonControlGroup RibbonGroup, .
, RibbonButton RibbonControlGroup ( ), RibbonControlSizeDefinition. WPF . -, . (, , ... , ).
, . RibbonControlGroup " ", 0. . , , , , , - .
: RibbonControlGroup, , :
<ribbon:RibbonGroup Header="Size at Control Level">
<ribbon:RibbonControlGroup>
<ribbon:RibbonControlGroup.ControlSizeDefinition>
<r:RibbonControlSizeDefinition ImageSize="Small" IsLabelVisible="False" />
</ribbon:RibbonControlGroup.ControlSizeDefinition>
<ribbon:RibbonButton SmallImageSource="Images\wizard-icon16.png" LargeImageSource="Images\wizard-icon32.png" Label="Button 1" />
<ribbon:RibbonButton SmallImageSource="Images\wizard-icon16.png" LargeImageSource="Images\wizard-icon32.png" Label="Button 2" />
<ribbon:RibbonButton SmallImageSource="Images\wizard-icon16.png" LargeImageSource="Images\wizard-icon32.png" Label="Button 3" />
<ribbon:RibbonButton SmallImageSource="Images\wizard-icon16.png" LargeImageSource="Images\wizard-icon32.png" Label="Button 4" />
</ribbon:RibbonControlGroup>
</ribbon:RibbonGroup>