Is the following an accurate description of "Thank You, M'am" by Langston Hughes? The woman is not a physically strong person; the would-be thief...

No, not quite, although you're getting close! Try reading the story a few more times, focusing on the details and imagining the characters in your mind. 

Let's take a close look at each of your descriptions:

1. "The woman is not a physically strong person."

No, this is not accurate. The woman in the story is described as "large," with the ability to pick the boy up by his shirt and shake him. This description hints that the woman is strong, able to lift a teenage boy up off the ground. The woman is also able to hold onto that boy even while he's desperate to get away, as well as drag him behind her as she walks. It's another hint that she's strong while he's weak.

2. "The would-be thief is a strong, tall person."

Strong, no; tall, maybe. Although the boy might be tall--the text doesn't seem to say for 100% certain whether he is or not--he's definitely not strong. The narrator describes the boy like this: "He looked as if he were fourteen or fifteen, frail and willow-wild, in tennis shoes and blue jeans." That word "frail" indicates that the boy is thin and weak, and "willow-wild" means he's like a moving willow tree, all thin and wispy. "Willowy" often means "tall," but that's not exactly the word that the author chose, so we can't be sure. But we do know that the boy must not weigh very much, since he fell over backwards when he grabbed onto the heavy purse.

3. "The story is about how the would-be thief becomes a good person."

Not quite, although that's pretty close. The story is more about how the would-be thief gets a chance (and the inspiration) to become a better person. The woman whose purse he tried to steal takes him home with her and gives him a good meal. She talks with him kindly, gives him enough money to buy the shoes he wants, and then lets him go home. That's when the story ends.

We do see some hints that the boy has decided to try being a better person: he listens to the woman, offers to run an errand for her, and thanks her for the money. He also thinks to himself that "he did not want to be mistrusted now." He wants to become a trustworthy person instead of someone who tries to steal from strangers.