TextBox.MaxLength Property

<Page  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
  <StackPanel>
    <TextBox 
      Text="Initial text in TextBox" 
      Width="200" 
      TextAlignment="Center"
      TextWrapping="Wrap" 
      MaxLength="500"
      MinLines="1" 
      MaxLines="5" />
  </StackPanel>
</Page>

StackPanel myStackPanel = new StackPanel();

//Create TextBox
TextBox myTextBox = new TextBox();
myTextBox.Width = 200;

// Put some initial text in the TextBox.
myTextBox.Text = "Initial text in TextBox";

// Set the maximum characters a user can manually type
// into the TextBox.
myTextBox.MaxLength = 500;
myTextBox.MinLines = 1;

// Set the maximum number of lines the TextBox will expand to 
// accomidate text. Note: This does not constrain the amount of 
// text that can be typed. To do that, use the MaxLength property.
myTextBox.MaxLines = 5;

// The text typed into the box is aligned in the center.
myTextBox.TextAlignment = TextAlignment.Center;

// When the text reaches the edge of the box, go to the next line.
myTextBox.TextWrapping = TextWrapping.Wrap;

myStackPanel.Children.Add(myTextBox);
this.Content = myStackPanel;