"Another'n at the the house that's field size" means "another child/sibling [another one] at the house who is now old enough/big enough to go into the field and help."
Earlier in the text, Atticus explains that Walter Cunningham is so poor that he can only pay Atticus in barter: nuts, firewood, turnip greens. Other professionals in the area accept the same kind of payment:
As the Cunninghams had no money to pay a lawyer, they simply paid us with
what they had. “Did you know,” said Atticus, “that Dr. Reynolds works the same
way? He charges some folks a bushel of potatoes for delivery of a baby."
Scout, too young to understand the humiliation of this kind of poverty, thinks this is normal. So when Walter says there's another sibling at home, Scout asks if the family paid the doctor for the baby's delivery with potatoes.
Atticus (and later the cook Calpurnia in the same scene) are chastising Scout for her bad manners.