So the following does the first part:
<Expander>
<Expander.Style>
<Style TargetType="Expander">
<Setter Property="IsExpanded" Value="False" />
<Setter Property="Header" Value="See More" />
<Style.Triggers>
<DataTrigger Binding="{Binding IsExpanded,RelativeSource={RelativeSource Self}}" Value="True">
<Setter Property="Header" Value="See Less" />
</DataTrigger>
</Style.Triggers>
</Style>
</Expander.Style>
</Expander>
But what if I want the header size to be 16pt too? Can't think of the correct syntax.