These are about 9 x 4-inches, having started with a 10-inch square; folded in half and filled with (dry) navy beans and two thimblefuls of dried lavender. In the past I've used other beans, pearl rice, flax seed, or legume.
Basically the filler material needs to be dry and something that can withstand heating in a microwave for a minute or two. Do not use ceramic balls or baking weights--they may ignite the cloth!
Another thing. If you use beans or rice, don't get them wet. I had a little mishap with a bean bag that got gathered up with the sheets when I washed them. What a surprise to find that it had expanded to the size of a brick and was just as dense, having been filled with pinto beans.
It was funny. We laughed hysterically at that bloated blob. I was just so grateful that the stitching had held together. And that it had not exploded inside the washer.
Oh my. A mess I don't even want to think about.