Связь ComboBox и TextBox в WPF

Давайте покажу, как менять шрифт глобально.

Вам нужно не выставлять шрифт конкретному элементу, а определить его на уровне окна. Для привязки нам понадобится пара простых конвертеров:

class RatioConverter : IValueConverter
{
    public double Min { get; set; }
    public double Max { get; set; }
    public double MinValue { get; set; }
    public double MaxValue { get; set; }
    public object Convert(object value, Type targetType, object p, CultureInfo ci)
    {
        double v = (double)value;
        var ratio = (v - Min) / (Max - Min);
        return MinValue + (MaxValue - MinValue) * ratio;
    }
    public object ConvertBack(object value, Type targetType, object p, CultureInfo ci) =>
        throw new NotSupportedException();
}
class MinConverter : IMultiValueConverter
{
    public object Convert(object[] values, Type targetType, object p, CultureInfo ci) =>
        values.Cast<double>().Min();
    public object[] ConvertBack(object value, Type[] targetTypes, object p, CultureInfo ci)=>
        throw new NotSupportedException();
}

Теперь сам XAML.

<Window x:Class="Test.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:sys="clr-namespace:System;assembly=mscorlib"
        xmlns:local="clr-namespace:Test"
        Title="Тест" Height="200" Width="300">
    <Window.Resources>
        <sys:Double x:Key="MinWidth">200</sys:Double>
        <sys:Double x:Key="MaxWidth">400</sys:Double>
        <sys:Double x:Key="MinHeight">150</sys:Double>
        <sys:Double x:Key="MaxHeight">300</sys:Double>
        <sys:Double x:Key="MinFontSize">15</sys:Double>
        <sys:Double x:Key="MaxFontSize">40</sys:Double>
        <local:RatioConverter
            x:Key="WidthConverter"
            MinValue="{StaticResource MinFontSize}" MaxValue="{StaticResource MaxFontSize}"
            Min="{StaticResource MinWidth}" Max="{StaticResource MaxWidth}"/>
        <local:RatioConverter
            x:Key="HeightConverter"
            MinValue="{StaticResource MinFontSize}" MaxValue="{StaticResource MaxFontSize}"
            Min="{StaticResource MinHeight}" Max="{StaticResource MaxHeight}"/>
        <local:MinConverter x:Key="MinConverter"/>
    </Window.Resources>
    <TextBlock.FontSize>
        <MultiBinding Converter="{StaticResource MinConverter}">
            <Binding Path="ActualWidth" RelativeSource="{RelativeSource Self}"
                     Converter="{StaticResource WidthConverter}"/>
            <Binding Path="ActualHeight" RelativeSource="{RelativeSource Self}"
                     Converter="{StaticResource HeightConverter}"/>
        </MultiBinding>
    </TextBlock.FontSize>
    <Window.MinWidth><StaticResource ResourceKey="MinWidth"/></Window.MinWidth>
    <Window.MaxWidth><StaticResource ResourceKey="MaxWidth"/></Window.MaxWidth>
    <Window.MinHeight><StaticResource ResourceKey="MinHeight"/></Window.MinHeight>
    <Window.MaxHeight><StaticResource ResourceKey="MaxHeight"/></Window.MaxHeight>
    <Grid>
        <TextBlock Text="Здравствуй, мир!"
                   HorizontalAlignment="Center" VerticalAlignment="Center"/>
    </Grid>
</Window>

Получается: