Is there a way to customize the default canvas size in Inkscape?
Sure. Just start with a blank document, change the canvas size to whatever you want, and then save the document as
templates/default.svg in your Inkscape config directory (
~/.config/inkscape on Linux). Then restart Inkscape, and it should open with whatever document you just saved as the default template.
You can also create your own document templates (which you can access from the File → New menu) by saving files with other names in that directory.
(Ps. If your interface language is set to something other than English, you may need to save the default template as
default.xx.svg instead of
xx stands for the two-letter ISO language code of your language.)