I just use a light tablespoon of filler in my packets when I make them. That tends to be the right amount. Also, not binding the packets as close to the filler as possible. Leaving a little space gives the packets a little give when they land, which generally makes them just that much safer.
Also, make sure there's no corn or sunflower seeds in your packet mix. It's a pain to sift them out if you buy the cheaper bags, but they'll get your packets rejected.
I'm unfamiliar with those... Millet tends to be the premium packet filling, which also tends to make up the bulk of seed mixes. Are they pokey? Are they big or small?
5 1/2 inches by 5 1/2 inches. 1 Tablespoon of birdseed. No sunflower seeds in the birdseed or other pointy seeds. A non stretchy fabric works best. I also use rainbow loom bands instead of rubber bands because they're smaller so they save time when making packets.
I actually prefer a slightly stretchy fabric. Old T-shirts are my favorite. I do 6 inch squares, small hair rubber bands and about a heaping tablespoon of seed. Play around a bit, see what feels good to you!