Textbox width change automatically

Hi again,

as Hiran wrote - use the TextWrapping-property to achieve that. In addition, set the MaxWidth-property accordingly. IOW:

<Window x:Class="WpfTests.Window20"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="Window20" SizeToContent="WidthAndHeight"><Grid Background="AntiqueWhite"><TextBox Text="Something to start with" Margin="10" MaxWidth="400" TextWrapping="Wrap"/></Grid></Window>