<Style TargetType="TextBlock">
    <Setter Property="Height" Value="26"/>
</Style>

<telerik:RadGridView Style="{x:Null}">
    ...
</telerik:RadGridView>

<Style TargetType="TextBlock">
    <Style.Triggers>
        <DataTrigger
            Binding="{Binding RelativeSource={RelativeSource Self},
            Converter={StaticResource HasAncestorTypeConverter},
            ConverterParameter={x:Type telerik:RadGridView}}"
            Value="False">

            <Setter Property="Height" Value="26"/>
        </DataTrigger>
    </Style.Triggers>
</Style>