Set the FontSize
property of the control itself and define an implicit TrimmedTextBlock
style that sets (preserves) the FontSize
of the header:
<xctk:PropertyGrid ... FontSize="20">
<xctk:PropertyGrid.Resources>
<Style TargetType="xctk:TrimmedTextBlock">
<Setter Property="FontSize" Value="12" />
</Style>
...
</xctk:PropertyGrid.Resources>
</xctk:PropertyGrid>