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.