The main character is round and dynamic because she is fully developed and undergoes a change.
A round character is one where many traits are described. We know that Sarah Penn is stubborn, intelligent, articulate, and can think divergently.
When looking at the barn and the house, Sarah feels frustrated. She sees that the barn is much bigger and nicer.
The house, standing at right angles with the great barn and a long reach of sheds and out-buildings, was infinitesimal compared with them. It was scarcely as commodious for people as the little boxes under the barn eaves were for doves.
The barn is where her husband puts his priorities. He does not even tell his wife that he is building another barn. This infuriates her. While telling her daughter that she will someday learn that she will never be the equal of men, she is clearly irritated and frustrated.
A dynamic character is one that undergoes a change. Sarah Penn is a good housewife at the beginning of the story. She listens to her husband and fulfills her role as a good wife, but when her husband breaks his promise to build her a nice house and instead decides to build himself a barn, she revolts. This demonstrates her potential for change.
When her husband starts building the barn, Sarah finally confronts him.
I ain't never complained, an' I ain't goin' to complain now, but I'm goin' to talk plain.
When that does not work, she moves into the barn. Even when the minister comes, she holds her ground. She does not back down, and demonstrates a new strength equal to men, showing her growth as a dynamic character.