The best way to do this unintuitive: you should set the canvas to the size you want using the "Page Setup" menu described in Jaden's answer then resize the image to fit the canvas. You don't resize the canvas to fit the image.
To prove that this works, I set the canvas to 150x150px in size and pasted a picture I have that is the same size (150x150px) into it. The image that appeard in the google drawing was smaller than the canvas. Turns out, google draw resized images that you paste into it. When I downloaded the image it's size was 150x150px, but the contents of the image had shrunk and had a white border around it. I then took the image in the google drawing, expanded it to fit the google canvas (the image snaps to the edge of the canvas when you bring it close enough), and redownloaded it. The download was an exact copy of the image I started with.