See the post for complete details.
I have two points to consider today. One is that doodling can be of any size. Some of the most elaborate and large scale examples of graffiti I've ever seen have been doodles. Some doodles rendered with a fine tipped pen have been almost microscopic.
Don't get wound up about size if you're looking to expand your definition of doodling, and thereby make it a useful tool for your journaling or sketching practice.
I think of both the images in today's post as doodles. They were both done one evening when I wanted to draw but it was late and I didn't want to get any supplies out. I didn't even want to go and get my journal. I picked up the date planner on my desk, which happens to be a Moleskine (I hate it because the weeks start on MONDAY and I like Sunday starts—I will never use one again; in fact I stopped using this one several times this year and used large wall calendars, but I digress).
Left: Facing page sketch. A quick doodle with fine point red pen, gray ink, black ink, then additions of pink (2 kinds of pink) and orange, just to see "what if."
The paper in this date planner, as you can see from the scans, is tissue thin. You can see the writing on the flipside of the page showing through. Between each week there are planning pages and it's on those pages that I sketched. I just picked up what was near by and sketched to see "what if?" What if I smudged the pencil shading with my finger, what if I used pink marker on the man's face. I didn't care about what the final result was because I was doodling with materials.
That brings me to my second point for today.
SKETCH ON ANYTHING.
It doesn't matter if it's crap paper (yes, crap paper, I didn't leave off a letter), just sketch if the mood takes you. Use a tool you're familiar with, or something that will work on that paper (I used pencil because the paper was fairly smooth but had a bit of tooth, and for the other sketch I used pen and ink and markers). Don't worry if things bleed through or get messed up. Don't worry if things aren't archival (because remember these are just doodles).
The only reason you need to worry is if you aren't sketching!