BitmapEncoder pngEncoder = new PngBitmapEncoder();
var finalBitmap = new RenderTargetBitmap(4000, 4000, 120, 120, PixelFormats.Pbgra32);
DrawingVisual dv = new DrawingVisual();

using (DrawingContext dc = dv.RenderOpen())
    {
        this.DrawTextBoxes(this.StackPanel1, dc, 0, 0);
        Console.WriteLine("height: " + this.TextBox1.RenderSize.Height);
        Console.WriteLine(this.TextBox1.FontSize.ToString());
        this.TextBox1.FontSize += 1;
        this.TextBox1.Text = "changed";
        Console.WriteLine(this.TextBox1.FontSize.ToString());
        Console.WriteLine("height: " + this.TextBox1.RenderSize.Height);
        this.DrawTextBoxes(this.StackPanel1, dc, 500, 0);
    }

finalBitmap.Render(dv);
pngEncoder.Frames.Add(BitmapFrame.Create(finalBitmap));

using (var file = File.OpenWrite("TestImage.png"))
   {
       pngEncoder.Save(file);
   }

public void DrawTextBoxes(UIElement element, DrawingContext dc, double x, double y)
    {
        var target = new RenderTargetBitmap((int)(element.RenderSize.Width/0.8), (int)(element.RenderSize.Height/0.8), 120, 120, PixelFormats.Pbgra32);
        target.Render(element);
        var frame = BitmapFrame.Create(target);
        dc.DrawImage(frame, new Rect(x, y, frame.Width, frame.Height));
    }

height: 20,31
15
16
height: 20,31