CtrEditor/ObjetosSim/ucBoton.xaml

<UserControl x:Class="CtrEditor.ObjetosSim.ucBoton"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:i="http://schemas.microsoft.com/xaml/behaviors"
             xmlns:ei="http://schemas.microsoft.com/xaml/behaviors"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
             xmlns:convert="clr-namespace:CtrEditor.Convertidores"
             mc:Ignorable="d">

    <UserControl.Resources>
        <convert:MeterToPixelConverter x:Key="MeterToPixelConverter"/>
    </UserControl.Resources>

    <Grid>
        <Border x:Name="BackgroundRectangle"
                BorderBrush="Black"
                BorderThickness="2"
                CornerRadius="0,0,10,10"
                Width="{Binding Tamano, Converter={StaticResource MeterToPixelConverter}}"
                Height="{Binding Tamano, Converter={StaticResource MeterToPixelConverter}, ConverterParameter=1.5}"
                VerticalAlignment="Top"
                HorizontalAlignment="Left"
                Background="Gray"/>
        <Ellipse Fill="{Binding ColorButton}"
                 Stroke="Black"
                 StrokeThickness="2"
                 Width="{Binding Tamano, Converter={StaticResource MeterToPixelConverter}}"
                 Height="{Binding Tamano, Converter={StaticResource MeterToPixelConverter}}"
                 HorizontalAlignment="Left"
                 VerticalAlignment="Bottom"
                 MouseLeftButtonDown="Ellipse_MouseLeftButtonDown"
                 MouseLeftButtonUp="Ellipse_MouseLeftButtonUp">
        </Ellipse>
    </Grid>
</UserControl>