I have a Google Drawings amongst my Google Docs files with a huge canvas. The shape I drew within the image is a lot smaller than the canvas though.
How can I tell Google Drawings automatically to shrink the size of the canvas to fit the size of my shape?
There is no way to do this automatically. The best way to do this would be to zoom to a level that allows you to quickly resize the canvas appropriately.
Go to View > Select zoom size (in or out or %) > use the drag-able corner in the bottom right of the canvas to resize the canvas.