You can set the size on the Canvas settings. Change to Size to ‘Specified’ and you get a box to input the width and height. You can enter in pixels or units of measure (in, cm, mm).
If there is a size you commonly use, create a custom menu with the gear menu.
Type in the size you want to save and select Add size to menu.
Then when you have new graphs, you can use the gear to access your custom size menu.
You can also create a Template by first creating the file that will serve as the template.
Place that file (or folder of files) in the menu location that you can access here.
When you save a file in this location, a preview pops-up you hover over the name.