This pair of transforms is called "tunable" because the Transform Control Panel has been unfrozen, allowing you to alter the transforms between runs. Click on the transform control panel (TCP) button to toggle the transform control panel into and out of view.

On the Control Panel you may enter new values for any of the transform parameters as well as add and delete transforms from the mix, using the transform checkboxes. Use this to experiment with creating new images.

You may enter new limits along the x and y dimensions directly in the limiting value boxes, since it is possible that the images your create may extend farther than the existing limits.