patching walls

Ha! Just a larger scale,that's all.

An electric hand sander helps the process.

I find that using a damp sponge smoothes the compound pretty well and minimizes the dust and debris to clean up.  You might use sand paper (or a dry-wall sanding pad) for the final coat.  

just the opposite anything larger than 1/20 is not good

With that "knock down" surface on the walls in this house, it is impossible to make a bad looking patch!  Just trowel on a bit of spackling, scrape it off and paint it, and you'll never see where it was done.  If it's a larger area, you can get tubes of "knock down" spackling, again just trowel it on, scrape it with a blade leaving high and low areas like the rest of the wall, paint and done!

Unlike modeling, free-handed, sloppy work is best here!

How big is the hole? Fist sized?  If its big I cut down the hole to a square or rectangle fit a piece of sheet rock in and get the plaster started. I use a belt sander to even it out. I used to fix them a lot as a teen because of a clutz in the house. I kept a sheet of gypsum board in the garage and cut pieces off to fill holes as needed. Be sure that your patch is nailed into a stud.