I made 8 or 9 Queen size biscuit quilts several years ago and 3 baby crib size. Base was 4" top 3" and I used several bags of polyester fibberfil per each one of the largest ones, probably 1# bags. I did stuff too many too full and they are extremely hard to sew the rows together. For this size of biscuit use about the size of a golf ball or ping pong ball, yours are larger and you would need to use more, not sure, maybe a tennis ball size amount. Sewing together into rows isn't that hard, when you sew 2 or more rows together is when you know if you stuffed them too much. I love the biscuit quilts, they are gorgeous when done. Good luck.
P.S. Keep the ball size loose. I would not use anything but polyester, these are heavy and even heavier when being washed, not easy to wash unless you have a very large washer.