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.