I use the large size for my adult female ball and she weighs in at 1900grams. She is about 3 years old. The hide is perfect for her - big enough for her to position herself any way she wants, but snug enough that she feels secure.
They are, however, big enough that I wouldn't recommend "growing" a baby into the large. They are large. The medium is acceptable for this purpose - then graduate the snake to the large when it outgrows the medium. :gj: