setting the parameters in graphic usercontrols

Your property setters actually need to do something. Computers are not magical, they only do what you tell them to. In this case, you need to tell the ellipses to resize themselves.

Try something like

class TwoCircles : UserControl {
 Ellipse e1, e2;
 public double A { { get { return e1.Width; } set { e1.Width = e1.Height = value; } }
 public double B { { get { return e2.Width; } set { e2.Width = e2.Height = value; } }
 
 public TwoCircles(double a, double b){
  e1=new Ellipse{Width=a,Height=a};
  e2=new Ellipse{Width=b,Height=b};
  // ... etc (position/add code)
 }
}

(I removed the a1 and b1 fields because all they were doing was storing parameters already available through the ellipses.)

MainPage xaml file to be used to demonstrate solution: Note the use of 3 sliders for the UC parameters
<UserControl
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="clr-namespace:setTest1"
x:Class="setTest1.MainPage"
Width="640" Height="480">
<Canvas x:Name="LayoutRoot" Background="White">
<TextBlock x:Name="aTxt" Height="28" Canvas.Left="323" TextWrapping="Wrap" Text="a:" Canvas.Top="158" Width="103"/>
<Slider x:Name="aSlider" Height="26" Canvas.Left="323" Canvas.Top="132" Width="123" LargeChange="10" Maximum="200" SmallChange="1" Value="100"/>
<TextBlock x:Name="bTxt" Height="28" Canvas.Left="463" TextWrapping="Wrap" Text="b: " Canvas.Top="158" Width="103"/>
<Slider x:Name="bSlider" Height="26" Canvas.Left="463" Canvas.Top="132" Width="123" LargeChange="10" Maximum="200" SmallChange="1" Value="100"/>
<TextBlock x:Name="dxTxt" Height="28" Canvas.Left="463" TextWrapping="Wrap" Text="dx:" Canvas.Top="216" Width="103"/>
<Slider x:Name="dxSlider" Height="26" Canvas.Left="463" Canvas.Top="190" Width="123" LargeChange="10" Maximum="400" SmallChange="1" Value="100" Minimum="100"/>
</Canvas>
</UserControl>

//twoCircles Usercontrol file
using System;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Documents;
using System.Windows.Ink;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Animation;
using System.Windows.Shapes;
namespace setTest1
{
public partial class twoCircles : UserControl
{
SolidColorBrush brA=new SolidColorBrush{Color=Colors.Brown};
SolidColorBrush brB=new SolidColorBrush{Color=Colors.Blue};
public Ellipse eA=new Ellipse(),eB=new Ellipse();
private double a1,b1,dx1;

public double a {
get {return a1;}
set {this.a1=value;
reDraw();
}
}

public double b {
get {return b1;}
set {this.b1=value;
reDraw();
}
}

public double dx {
get {return dx1;}
set {this.dx1=value;
reDraw();
}
}
public twoCircles()
{
InitializeComponent();
eA.Fill=brA;
eB.Fill=brB;
LayoutRoot.Children.Add(eA);
LayoutRoot.Children.Add(eB);
}
private void reDraw()
{
eA.Width=a;
eA.Height=a;
eB.Width=b;
eB.Height=b;
eB.Margin=new Thickness(dx,(a-b)/2,0,0);
}
}
}

//MainPage.cs file:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Net;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Animation;
using System.Windows.Shapes;
namespace setTest1
{
public partial class MainPage : UserControl
{
twoCircles tC=new twoCircles();
static double a=200,b=100,dx=200;
public MainPage()
{
InitializeComponent();
tC.a=a;tC.b=b;tC.dx=200;
Canvas.SetLeft(tC,200);Canvas.SetTop(tC,300);
LayoutRoot.Children.Add(tC);
initSliders();
}
private void initSliders()
{
aSlider.ValueChanged += new RoutedPropertyChangedEventHandler<double>(aSlider_ValueChanged);
aSlider.Value=a;
aTxt.Text="a: " +a.ToString("000");
bSlider.ValueChanged += new RoutedPropertyChangedEventHandler<double>(bSlider_ValueChanged);
bSlider.Value=b;
bTxt.Text="b: " +b.ToString("000");
dxSlider.ValueChanged += new RoutedPropertyChangedEventHandler<double>(dxSlider_ValueChanged);
dxSlider.Value=dx;
dxTxt.Text="dx: " +dx.ToString("000");
}
private void bSlider_ValueChanged(object sender, RoutedPropertyChangedEventArgs<double> e)
{
LayoutRoot.Children.Remove(tC);
b=bSlider.Value;
tC.b=b;
bTxt.Text="b: " +b.ToString("000");
LayoutRoot.Children.Add(tC);
}
private void aSlider_ValueChanged(object sender, RoutedPropertyChangedEventArgs<double> e)
{
LayoutRoot.Children.Remove(tC);
a=aSlider.Value;
tC.a=a;
aTxt.Text="a: " +a.ToString("000");
LayoutRoot.Children.Add(tC);
}
private void dxSlider_ValueChanged(object sender, RoutedPropertyChangedEventArgs<double> e)
{
LayoutRoot.Children.Remove(tC);
dx=dxSlider.Value;
tC.dx=dx;
dxTxt.Text="dx: " +dx.ToString("000");
LayoutRoot.Children.Add(tC);
}
}
}