Set Stroke, StrokeThickness, Height and Width for Ellipse
<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="350" Width="300">
<UniformGrid Columns="3" HorizontalAlignment="Center" VerticalAlignment="Center">
<Ellipse Stroke="Black" StrokeThickness="3" Height="100" Width="70"/>
</UniformGrid>
</Window>
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