Changing font size of a button - wpf

Question

In my .xaml file these are my resources:
<DataTemplate DataType="{x:Type vm:KeyModel}">
<TextBlock DataContext="{Binding RelativeSource={RelativeSource TemplatedParent}}" FontFamily="Verdana" FontSize="26" Margin="0" >
<Run Text="{Binding Content.Text, Mode=OneWay}" />
</TextBlock>
</DataTemplate>
And here's the style:
<Style x:Key="RoundCorner" TargetType="{x:Type Button}">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type Button}">
<Border CornerRadius="8" BorderBrush="#006AB6" BorderThickness="1" Name="border" Background="{TemplateBinding Background}">
<Grid x:Name="grid" >
<ContentPresenter HorizontalAlignment="Center" VerticalAlignment="Center"></ContentPresenter>
</Grid>
</Border>
<ControlTemplate.Triggers>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
<Setter Property="FontSize" Value="18"/>
</Style>
And in code I have:
<Button Name="btn_Q" FontSize="8" Content="{Binding KB.Key_Q}" Grid.Column="1" Style="{DynamicResource RoundCorner}"/>
<Button Name="btn_A" Content="{Binding KB.Key_A}" Grid.Column="1" Style="{DynamicResource RoundCorner}"/>
But the FontSize for btn_Q doesn't get to be 8. How can I fix it? I need to make only btn_Q to have a fontsize of 8. All of my other buttons, like btn_A are fine with the style's fontsize.