How do I change the Dock shape to an Ellipse?
  


By default, Circle Dock is quite literally circular in shape; hence its name. However you can change the shape to an Ellipse (or oval) simply by selecting "Ellipse" from the Shape combo box which you'll find on the Dock Shape page of the main Properties dialog.

Having selected "Ellipse", you'll probably also want to modify its height and width - actually you are modifying the innermost "ring"'s height and width. As you do so, the background will be redrawn to the correct aspect ratio.

You may also want to adjust the Separation between Circles value to strike a compromise between avoiding "collisions" at the top and bottom of the Dock and, at the same time, reducing the distance between rings at the widest parts. Generally speaking, the flatter the ellipse, the greater the ring separation value should be.

Caveat
Painting an elliptical dock does take longer than painting a circular dock due to the more complex triganometry involved.

Note
Depending on the aspect ratio of your Elliptical Dock (ie its flatness) items will appear closer together when positioned close to the major axis (normally the left and right-most points). In fact, all the items are spaced equally along the circumference of the Ellipse which you can verify for yourself by making the height and width of the Ellipse the same values.