Thanks for the samples. Unfortunately, they don't address the need to specify an ellipse width in METERS, or even degrees of longitude - you still seem to be specifying the width of the ellipse in pixels.
This is what I came up with. I found another post somewhere on the forum describing how to go from a pixel distance on the map to the degrees longitude equivalent, then worked back from there. My view model for a map object now has this property:
var latRad = Location.Latitude * Math.PI / 180;
var lengthOfOneDegLong = (Math.PI/180)*EarthRadius*Math.Cos(latRad);
var width = Accuracy/lengthOfOneDegLong;
var pixelWidth = TileSize*Math.Pow(2.0, ZoomLevel)*(width/360);
It's not pretty, and I'm not 100% certain it's correct, but the circles that are drawn do appear to be the correct size, or at least near enough to be acceptable.