public partial class ProportionalUserControl : UserControl
    {
        public ProportionalUserControl()
        {
            SizeChanged += ProportionalUserControl_SizeChanged;
            InitializeComponent();
        }

        private void ProportionalUserControl_SizeChanged(object sender, SizeChangedEventArgs e)
        {
            resize(e.NewSize.Width, e.NewSize.Height);
        }

        /// <summary>Отношение ширины к высоте - Width/Height</summary>
        public double RatioWidthToHeight
        {
            get { return (double)GetValue(RatioWidthToHeightProperty); }
            set { SetValue(RatioWidthToHeightProperty, value); }
        }

        // Using a DependencyProperty as the backing store for RatioWidthToHeight.  This enables animation, styling, binding, etc...
        public static readonly DependencyProperty RatioWidthToHeightProperty =
            DependencyProperty.Register("RatioWidthToHeight", typeof(double), typeof(ProportionalUserControl), new PropertyMetadata(1.0));

        protected void resize(double width, double height)
        {
            if (width > height * RatioWidthToHeight)
            {
                Width = height * RatioWidthToHeight;
                Height = height;
            }
            else
            {
                Width = width;
                Height = width / RatioWidthToHeight;
            }
        }
    }

[/SRC] [SRC XML]<UserControl x:Class="WPF1903.ProportionalUserControl"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
             xmlns:local="clr-namespace:WPF1903"
             mc:Ignorable="d" 
             d:DesignHeight="450" d:DesignWidth="800">
    <Grid x:Name="PART_Grid" Background="AliceBlue">
            
    </Grid>
</UserControl>

<Window x:Class="WPF1903.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:WPF1903"
        xmlns:lib="clr-namespace:CommLibrary;assembly=CommLibrary"
        mc:Ignorable="d" Focusable="True"
        Title="MainWindow" Height="450" Width="800">
    <StackPanel>
        <Border x:Name="border" Width="400" Height="200" BorderThickness="2" BorderBrush="Coral" Padding="5" Margin="5">
            <local:ProportionalUserControl />
        </Border>
        <TextBox Text="{Binding Width, ElementName=border}"/>
        <TextBox Text="{Binding Height, ElementName=border}"/>
    </StackPanel>
</Window>

    public partial class ProportionalUserControl : UserControl
    {
        public ProportionalUserControl()
        {
            SizeChanged += ProportionalUserControl_SizeChanged;
            InitializeComponent();
            ren
        }

        private void ProportionalUserControl_SizeChanged(object sender, SizeChangedEventArgs e)
        {
            resize(e.NewSize.Width, e.NewSize.Height);
        }

        /// <summary>Отношение ширины к высоте - Width/Height</summary>
        public double RatioWidthToHeight
        {
            get { return (double)GetValue(RatioWidthToHeightProperty); }
            set { SetValue(RatioWidthToHeightProperty, value); }
        }

        // Using a DependencyProperty as the backing store for RatioWidthToHeight.  This enables animation, styling, binding, etc...
        public static readonly DependencyProperty RatioWidthToHeightProperty =
            DependencyProperty.Register("RatioWidthToHeight", typeof(double), typeof(ProportionalUserControl), new PropertyMetadata(1.0));

        protected void resize(double width, double height)
        {
            if (width > height * RatioWidthToHeight)
            {
                PART_Grid.Width = height * RatioWidthToHeight;
                PART_Grid.Height = height;
            }
            else
            {
                PART_Grid.Width = width;
                PART_Grid.Height = width / RatioWidthToHeight;
            }
        }

    }

    public partial class ProportionalUserControl : UserControl
    {
        public ProportionalUserControl()
        {
            //SizeChanged += ProportionalUserControl_SizeChanged;
            InitializeComponent();
        }

        private void ProportionalUserControl_SizeChanged(object sender, SizeChangedEventArgs e)
        {
            resize(e.NewSize.Width, e.NewSize.Height);
        }

        /// <summary>Отношение ширины к высоте - Width/Height</summary>
        public double RatioWidthToHeight
        {
            get { return (double)GetValue(RatioWidthToHeightProperty); }
            set { SetValue(RatioWidthToHeightProperty, value); }
        }

        // Using a DependencyProperty as the backing store for RatioWidthToHeight.  This enables animation, styling, binding, etc...
        public static readonly DependencyProperty RatioWidthToHeightProperty =
            DependencyProperty.Register("RatioWidthToHeight", typeof(double), typeof(ProportionalUserControl), new PropertyMetadata(1.0));

        protected void resize(double width, double height)
        {
            if (width > height * RatioWidthToHeight)
            {
                PART_Grid.Width = height * RatioWidthToHeight;
                PART_Grid.Height = height;
            }
            else
            {
                PART_Grid.Width = width;
                PART_Grid.Height = width / RatioWidthToHeight;
            }
        }

        protected override Size MeasureOverride(Size constraint)
        {
            double width, height;
            if (constraint.Width > constraint.Height * RatioWidthToHeight)
            {
                width = constraint.Height * RatioWidthToHeight;
                height = constraint.Height;
            }
            else
            {
                width = constraint.Width;
                height = constraint.Width / RatioWidthToHeight;
            }

            return new Size(width,height);
        }
    }

    public partial class ProportionalUserControl : UserControl
    {
        public ProportionalUserControl()
        {
            InitializeComponent();
        }

        public double RatioWidthToHeight
        {
            get { return (double)GetValue(RatioWidthToHeightProperty); }
            set { SetValue(RatioWidthToHeightProperty, value); }
        }

        // Using a DependencyProperty as the backing store for RatioWidthToHeight.  This enables animation, styling, binding, etc...
        public static readonly DependencyProperty RatioWidthToHeightProperty =
            DependencyProperty.Register("RatioWidthToHeight", typeof(double), typeof(ProportionalUserControl), new PropertyMetadata(1.5));

        protected override Size MeasureOverride(Size constraint)
        {
            double width, height;
            if (constraint.Width > constraint.Height * RatioWidthToHeight)
            {
                width = constraint.Height * RatioWidthToHeight;
                height = constraint.Height;
            }
            else
            {
                width = constraint.Width;
                height = constraint.Width / RatioWidthToHeight;
            }

            return new Size(width, height);
        }

    <StackPanel>
        <Border x:Name="border" Width="400" Height="200"
                Padding="5" Margin="5" BorderThickness="2" BorderBrush="LightGreen" >
            <Border BorderThickness="2" BorderBrush="Coral" VerticalAlignment="Center" HorizontalAlignment="Center" >
                <local:ProportionalUserControl VerticalAlignment="Center" HorizontalAlignment="Center" />
            </Border>
        </Border>
        <TextBox Text="{Binding Width, ElementName=border}"/>
        <TextBox Text="{Binding Height, ElementName=border}"/>
    </StackPanel>