I was taught that doodling is to build visual memory, not muscle memory. That makes more sense to me. Understanding what something will look like when connected altogether is something that doesn't always come naturally to people & that's where doodling helps. If I can draw it, then I can figure out how to get it on a quilt; however, it does not start to come automatically to me (muscle memory) until I've actually done the motion a number of times (i.e., practiced on an actual quilt/quilt sandwich).

One way I know some people build muscle memory without working with actual fabric is to draw the exact design they want on a sheet of paper -- you can do larger designs by buying tracing paper on a roll. Then, they will tape a laser pointer to a stand or a light fixture or something so it's pointing down and then will hold the piece of paper like it's a quilt & move it so the laser pointer traces the lines on the paper. You are lacking the weight of fabric & batting, but at least you're doing the actual movement for FMQ.

I rarely worry much about scale when I'm doodling. When I draw an original design, I will use layers of tracing paper to figure out what quilting designs I want at the actual (or much closer to the actual) size of the quilt. But other than that, I just practice the motion of how the line transitions from one part of the quilting design to the next with doodling. If I have a tricky part, I will draw it out with marker on a practice sandwich & practice quilting on my machine. I do find that all of that helps me.

I use a dry erase board to practice designs. when I was first trying fmq i bought a pkg of felt squares to practice on rather than making "sandwiches". I think it helped co-ordinate my hands and I didn't feel so "wasteful"

I haven't really gotten into the idea of practicing FMQ designs on paper because my drawing with a pencil is worse than my FMQ. I would have never quilted anything. And then, I wanted to do feathers! After much research on line and buying a couple of books, I did practice the feathers on paper and found it very helpful. Really worked for me on feathers.

I don't doodle to scale either...sometimes I use a clipboard with scrap paper or a dry erase board from my sewing room..other times I'll just doodle on whatever is handy. About the only thing that comes close to actual size is when I practice drawing feathers. I mostly FMQ on my DM but I feel the doodling is still helpful.

I also use a Boogie Board. You can doodle all over then push the button and it's gone. I got mine for under $25 and keep it close to my Sweet 16. Then I can doodle for a while, do a practice piece, and quilt it on my quilt top. My Boogie Board is probably a 4X6 surface area.

I've been using my ipad and a free app called SketchBook to doodle with. I tap "undo" instead of "clear" to avoid the ad flood attached to "clear". My GC like to play with the app too. :)

I doodle on dry erase to. Mostly just for fun and to remember designs I want to try on quilts. I like doodling in different sizes on purpose, because it's much more natural for me to doodle small, but I have to almost 'force' myself to make the shapes real large. I actually have to think about making it bigger. As I doodle, the shapes start leaning and shrinking naturally.
SketchBook is fun too...so is scrap paper...

The repetitive motion is what gives you the mental motion. Any scale you draw in is okay, small or large, the more you do it the motion the better you get at repeating it in any size. It is nice to play rhythmic music while skating across the paper.

Any size I have room for on my white board. Doodling doesn't so much work on muscle memory if you are a sit down quilter but it does help you figure out how to move around your space without trapping yourself in a corner. In my opinion, anyway.

I use a sit down but still feel doodling really helps even though I move the fabric rather than the needle... don't tell my pastor but I often doodle in church