If you want the labels in the GroupBox to be smaller and everything else in the group field to fit the size of the text of the GroupBox header, use the style:
<GroupBox FontSize="14" Header="Header Text">
<GroupBox.Resources>
<Style TargetType="Label">
<Setter Property="FontSize" Value="8" />
<Setter Property="FontWeight" Value="Normal" />
</Style>
</GroupBox.Resources>
<StackPanel>
<Label Text="Label Text" />
<Label Text="Another Label" />
<TextBlock Text="This will match the group header" />
</StackPanel>
</GroupBox>
, GroupBox GroupBox, TextBlock :
<GroupBox>
<GroupBox.Header>
<TextBlock Text="Header Text" FontSize="14" />
</GroupBox.Header>
<StackPanel>
<Label Text="Label Text" />
<Label Text="Another Label" />
<TextBlock Text="This will be the default font" />
</StackPanel>
</GroupBox>
, GroupBox, ( ) GroupBox.