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.)
A short answer for Windows users:
Open any document, you may want to start from the default document or another template using
File | Templates.
Document Properties(Shift-Ctrl-D) you want on the document using tabs, This is where you find the document size, the size units, the grid (default) units, etc,
Select user interface options (like snap option buttons),
Add any default objects in the document if you need it,
Save this document as
This folder is actually
C:\Users\Me\AppData\Roaming\inkscape\templates in Windows 7, and is empty.
I'll always be surprised by a software made for the entire world which is configured to use imperial units by default, and which has a site explaining how to change the units by referring to Linux. Nearly the entire world uses the SI units and Windows... But that's free and open source software, so this small annoyance is largely balanced by the rest.
On top of the other reply, you can save much more changes to templates: snapping options, guides etc. This article explains that in details.