If you open up a large canvas (e.g. A3 600dpi) then zoom in, you get a massive space to sketch in. As you’re zoomed in it should perform well, and the file size will reflect the content - so if you only have a few sketches in the middle of the canvas, it won’t take up loads of space when you save.
If you combine this with a minimal interface, such as you can achieve with the Kanvasbuddy and Redesign plugins, it makes for a really nice sketching environment with all the power of Krita still available.
It might be worth reading the this thread I wrote a year ago, requesting an infinite canvas feature.
And I did a mockup in a similar thread by EyeOdin to demonstrate what I described above.