You can do this with a "coords" or "donecanvasdialog" message sent to the canvas (ie. using [iemguts/sendcanvas] or [send pd-canvasname.pd]).

I don't remember the arguments exactly, but you can figure them out by opening a .pd file with GOP enabled and then searching for the "coords" line at the bottom. There are 5 or 6 arguments which correspond to GOP dimensions.