Sometimes the thumbnail sketches and planning take quite awhile, whether the work is commissioned or not, though I find that once I stretch and gesso a canvas, the process is a little quicker. I tend to be able to envision the image prior to applying it on the canvas and I think it is due to the act of physically seeing the substrate I'll be working on, both it's size and shape, which helps me to decide on the final drawing. I'm glad that I was taught to plan an image by utilizing the entire space instead of needing to crop work - working on paper that's not as big of an issue, unless you have a specific frame shape and size in mind, but working on stretched canvas the composition needs to feel 'right' from the beginning.
Years ago I received pivotal guidance from an artist by the name of Audrey Mabee whose work and character I admire immensely. She suggested I sketch directly on canvas, using paint instead of pencils (or charcoal, which I was using at the time), in order to work more intuitively and loosely. Since then, it is my preferred method of working. Though I still like to create thumbnail sketchtes in my journal to plan the composition a bit, including finalizing the perspective I plan to use.
I cannot imagine creating a full small painting and then working a larger one from that, as members of the Group of 7 or many professional artists. I always feel I'd loose that initial 'freshness' and that the work would become another job, be labour-intensive, instead of the joy of creation that it is...which is ultimately the reason I paint in any case. I love the joy of creating something that has never been before, often surprising myself in the process. I love the sheer abandon of the act of painting when everything else in my life is quite structured (by choice). And I love it when others react positively to what has been funneled through me as I am only a vessel and for that I am immensely grateful.