As you said, there are lots of way of doing this.
For starter, you can have a style template and then use StaticResource to get the style for it. For example:in your view mainwindow.xaml
<Window.Resources>
<Style TargetType="{x:Type Control}" x:Key="baseStyle">
<Setter Property="FontSize" Value="100" />
</Style>
<Style TargetType="{x:Type Button}" BasedOn="{StaticResource baseStyle}"></Style>
<Style TargetType="{x:Type Label}" BasedOn="{StaticResource baseStyle}"></Style>
<Style TargetType="{x:Type TextBox}" BasedOn="{StaticResource baseStyle}"></Style>
<Style TargetType="{x:Type ListView}" BasedOn="{StaticResource baseStyle}"></Style>
<!-- ComboBox, RadioButton, CheckBox, etc... -->
</Window.Resources>
OR Another way is:
View: mainwindow.xaml
<Window>
<Window.resources>
<Style TargetType="{x:Type TextBox}">
<Setter Property="FontSize"
Value="14" />
</Style>
</Window.resource>
<Grid>
<TextBox Text="blah"/>
</Grid>
</Window>
OR
you can do it from mainwindow constructor
Style = (Style)FindResource(typeof(Window));
and this, you will put this in the app.xaml
<Style TargetType="{x:Type Window}">
<Setter Property="FontSize"
Value="14" />
</Style>