You should look in PDF::API2::Content for information on image(), transform(), and related methods. In particular, look at the "External Objects" and "Coordinate Transformations" sections.

The '0, 0' to which you referred are co-ordinates, not sizes. As the code already sets the co-ordinates via the '-translate' option of transform(), further modification of the co-ordinates by image() is not wanted.

The image() method has a number of forms allowing you to change width, height, and scale. There's also a '-scale' option for transform(). Note that if you change the size, you'll also need to adjust the co-ordinates accordingly.