scale rotating boundingbox

scale rotating boundingbox

I recently started to develop a Windows Phone game with XNA. I have problem as you might have guessed collision detection. After looking up tutorials about all the types that can be achieved I decided I will go for the basic rectangular collision detection. I have a rotating sprite and a method that calculates the bounding box every time in the Update() method so I know where it's bounding box is then I simply check for intersection between all the lines of the box with all the lines of the other sprite's boxes. But since my box is appearing square shaped and my texture of that rotating sprite is Rectangular I wanna scale the bounding box so it will be closer to the texture's size. Here is what I have for calculating the corners of the rotating bounding box:
        double baseAngle = Math.Atan(this.Height / this.Width);
        double len = Math.Sqrt(this.Height * this.Height / 4 + this.Width * this.Width / 4);

        Vector2 tr = new Vector2((float)(Math.Sin(baseAngle + this.Rotation) * len) + this.Position.X, (float)(Math.Cos(baseAngle + this.Rotation) * len) + this.Position.Y);
        Vector2 tl = new Vector2((float)(Math.Sin(Math.PI - baseAngle + this.Rotation) * len) + this.Position.X, (float)(Math.Cos(Math.PI - baseAngle + this.Rotation) * len) + this.Position.Y);
        Vector2 bl = new Vector2((float)(Math.Sin(Math.PI + baseAngle + this.Rotation) * len) + this.Position.X, (float)(Math.Cos(Math.PI + baseAngle + this.Rotation) * len) + this.Position.Y);
        Vector2 br = new Vector2((float)(Math.Sin(2 * Math.PI - baseAngle + this.Rotation) * len) + this.Position.X, (float)(Math.Cos(2 * Math.PI - baseAngle + this.Rotation) * len) + this.Position.Y);`
any help would be appreciated. Thanks