If you animating the node's RenderTransform, that won't change Bounds. The width and height of the rendered node at the end of the animation should be the old width and height multiplied by the final value of the animation. Assuming you are using the sample code from a few weeks ago, the centers of the old and new rectangles should coincide, so you can find the rectangle like this

Rect r = node.Bounds;
Point c = new Point(r.X + r.Width / 2, r.Y + r.Height / 2);
double w = r.Width * finalScaleX;
double h = r.Height * finalScaleY;
Rect renderedRect = new Rect(c.X - w / 2, c.Y - h / 2, w, h);
 

I hope that helps,
Stoyan