Text box control in Silverlight supports multi line mode but surprisingly it does not have such property as Multyline. That is why, in order to make one text box multi line, you have to set its property AcceptsReturn to true.
Xaml
<TextBox x:Name="textBox" AcceptsReturn="True"></TextBox>
C#
TextBox textBox = new TextBox();
textBox.AcceptsReturn = true;
That's it!