lengthProperty

The preferred length of the bar. i.e. the preferred width for this node, when orientation == horizontal, and the preferred height when vertical.

If you set growPriority, and place this inside a Node which changes size, then the ProgressBar will also change length.

The preferred size in the other direction (i.e. the thickness) is governed by the Theme.

If you don't like the default thickness, then create your own theme (combined with Tantalum), and set the preferred Width/Height and the radius of the backgrounds appropriately. FYI, It is possible to change the thickness on an ad-hoc basis (without using a Theme), but it isn't recommended.

As this is Stylable, you can have the Theme set the default length. Tantalum set the length to 200.