When I was making last night's video, I thought to myself "maybe some people will wonder how Inkscape always starts up with a 12x12 page on my computer" and thought I might do a short video on how to set that up. Then today, someone asked about that very thing, so I knew it was a good idea! :)
UPDATE April 2015
I received a comment on this post today, saying that this technique no longer works on Inkscape v. 0.91 (the current latest version at time of writing). I have stuck with version 0.48 ever since I installed it so I thought I'd better install the new version and check it out for myself! I installed v. 0.91 and the process for setting up the default page size to 12", as shown in the video below, still works just fine as far as I can see, it's worked for me anyway.