Set Border Margin, BorderThickness, BorderBrush, Width and Height
<Window x:Class="WpfApplication1.Window1"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="WPF" Height="160" Width="300">
<Grid>
<Border Margin="8"
BorderThickness="1"
BorderBrush="Black"
Width="160"
Height="60">
<TextBlock Foreground="DarkGray"
VerticalAlignment="Center"
HorizontalAlignment="Center"
ToolTip="This is a custom tooltip"
Text="Mouse Over for tooltip"/>
</Border>
</Grid>
</Window>
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