Пример 1

gistfile1.cs

using System.Windows.Controls;

namespace TerstWpfApp
{
    /// <summary>
    /// Логика взаимодействия для MyUserControl.xaml
    /// </summary>
    public partial class MyUserControl : UserControl
    {
        
        public MyUserControl()
        {
            InitializeComponent();
        }

        public double TextBlockWidth
        {
            get { return TextBox.Width; }
            set { TextBox.Width = value; }
        }
    }
}

gistfile2.xml

<UserControl x:Class="TerstWpfApp.MyUserControl"
             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"
             mc:Ignorable="d"
             d:DesignHeight="300" d:DesignWidth="300">
  <Grid>
      <TextBox Name="TextBox"></TextBox>
  </Grid>
</UserControl>

gistfile3.txt

<Window x:Class="TerstWpfApp.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:terstWpfApp="clr-namespace:TerstWpfApp"
        Title="MainWindow"
        Height="350"
        Width="525">
    
    <Grid>
        <terstWpfApp:MyUserControl Name="MyUserControl">
          
        </terstWpfApp:MyUserControl>
    </Grid>
</Window>

gistfile4.cs

using System.Windows;

namespace TerstWpfApp
{
    /// <summary>
    /// Логика взаимодействия для MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        public void Tmp()
        {
            this.MyUserControl.TextBlockWidth = 1234.0;
        }
    }
}