I haven't tried this, and it is just an idea from the top of my head. For the width and height of the Ellipse, you could try to create a converter as using the following binding:
<esriSymbols:MarkerSymbol x:Name="MarkerSymbol_EOL">
<esriSymbols:MarkerSymbol.ControlTemplate>
<ControlTemplate>
<Canvas>
<Ellipse Height="{Binding Path=Resolution, ElementName=MyMap}, Converter={StaticResource SizeConverter}, ConverterParameter={Binding ElementName=MyMap}"
...
SizeConverter takes in the resolution of the map which will trigger an update when the zoomlevel changes. As a parameter, pass in the map.Inside of the converter, try to use the maps MinimumResolution, MaximumResolution and Resolution to calculate the actual width and height for the ellipse. You probably might want to add some logic for levels so the symbol won't disappear or cover the whole map when zooming too far in or out.Bjørnar Sundsbø