Explain "field size" and "bushel of potatoes" in ‘To Kill A Mockingbird’

From chapter 3 of To Kill A Mockingbird by Harper Lee:

[Walter Cunningham] ‘Reason I can’t pass the first grade, Mr Finch, is I’ve had to stay over ever’ spring an’ help Papa with the choppin’, but there’s another’n at the house now that’s field size’

‘Did you pay a bushel of potatoes for him?’ I asked, but Atticus shook his head at me.

What do the sentences in bold mean?

"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.

• In case it's not clear, a "bushel" is a unit of measurement in agriculture that varies from crop to crop. For potatoes, it's about 50 pounds. Again, depending on the crop, it may have been a container about the size of 8 gallons, but weight is used exclusively these days. Jan 21, 2019 at 19:44