using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace WpfApplication1
{
    public class Class1: INotifyPropertyChanged
    {
        public SizeConverter rem
        {
            get;
        }
        public Class1()
        {
            rem = new SizeConverter();
        } 
        public event PropertyChangedEventHandler PropertyChanged;
        public void RaiseSizeChanged()
        {
            if (PropertyChanged != null)
            {
                PropertyChanged(this, new PropertyChangedEventArgs("rem"));
            }
        }
    }

    public class SizeConverter : Dictionary<string, double>
    {
        public static double S = 1;
        public new double this[string key]
        {
            get
            {
                return Convert.ToInt32(key) / S;
            }
            set
            {

            }
        }


    }
}

<Window x:Class="WpfApplication1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfApplication1"
        mc:Ignorable="d"
        Title="MainWindow" Height="350" Width="525">
    <Window.Resources>
        <local:Class1 x:Key="a"></local:Class1>
    </Window.Resources>
    <Grid>
        <Label x:Name="lbl" Content="hello" FontSize="{Binding Source={StaticResource a},Path=rem[80]}"></Label>
        <Button Content="click" Click="Button_Click" Margin="140,201,203,49"></Button>
    </Grid>
</Window>

private void Button_Click(object sender, RoutedEventArgs e)
        {
            SizeConverter.S = 2;
            ((Class1)Resources["a"]).RaiseSizeChanged();
        }