Inkscape: Changing default canvas size

Since Inkscape 1.0 there’s an easy way to do this:

1. Create a new blank document (File → New).
2. Open the File → Document Properties… dialog and set your canvas size (and any other settings you want to change the defaults for, like display units, guides, grids, background color, etc.) the way you want it to be.
3. Now open the File → Save Template… dialog, enter whatever you want as the name of the template, make sure the “Set as default template” checkbox is checked and press Save.

^{Credit to Spencer Russell for pointing this out. Please go upvote their answer if you found this useful.}

The older way, which should still work, is to 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 from Template dialog) 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 default.svg, where xx stands for the two-letter ISO language code of your language. This should not be necessary in recent versions of Inkscape, however.)