Unfortunately, there is no physical description of Lady Macbeth. The closest the reader gets is when Duncan flatters her as a "fair (i.e. beautiful) and noble hostess" in Act I, Scene 6. It is important to remember that Shakespeare was a playwright, and did not need to include the detailed physical description of his characters that a novelist would. However, Shakespeare clearly intends to juxtapose Lady Macbeth's ruthlessness and ambition with contemporary notions of femininity, and this contrast would have been more marked if Lady Macbeth was portrayed as beautiful and highly feminine in appearance.
Perhaps, as the doppelganger of Macbeth--she calls him "my dearest partner of greatness"--the physical description of Lady Macbeth is insignificant. Certainly, the emphasis of character development of Lady Macbeth lies in the tragedy of her imagination that transports her into the preternatural desire to rid her husband of "the milk of human kindness" by taking the milk from her "woman's breasts" for gall. She embraces male aggression and cruelty in her desire for power.
Their mutual lust for power assimilates Lady Macbeth into Macbeth. But, their assimilation is not complementary. As long as he has a conscience, she sacrifices hers; when Macbeth loses his conscience and embraces complete tyranny, Lady Macbeth is ridden with guilt and her conscience is so tortured that she kills herself out of her guilt, dissolving into the "walking shadow" of Macbeth.
Lady Macbeth is ambitious, manipulative, cruel and unstable. There is not much about her physical appearance. She is described simply as Macbeth’s wife, but her words speak volumes about her personality. We can deduce that Lady Macbeth is a very feminine looking, beautiful woman but she behaves very harshly. In Act I, Scene VII, Macbeth comments on the contradiction between her behavior and her looks.
Bring forth men-children only,
For thy undaunted mettle should compose
Nothing but males. (enotes etex pdf, p. 24)
Lady Macbeth comments herself that she has small hands (p. 77). Duncan addresses her as “fair and noble hostess” (p. 22) in Act 1, Scene 6.
When we first meet her in Act I, Scene 5, she is reading a letter from Macbeth telling her about the prophecies and his promotion. Rather than be pleased, she worries that he does not have what it takes to become king.
Yet do I fear thy nature;
It is too full o’ the milk of human kindness(15)
To catch the nearest way. Thou wouldst be great;
Art not without ambition, but without
The illness should attend it. (p. 19)
She worries that Macbeth is too kind, and not ambitious enough to do something like kill the king. In Act I, Scene 7, she gets angry when Macbeth worries about failing and responds, “But screw your courage to the sticking-place/And we'll not fail” (p.24) and “look like the innocent flower/But be the serpent under't (p. 18). She continually manipulates him until he kills Duncan and becomes King himself. Then she seems to have second thoughts, not wanting him to commit more murders. In Act III, Scene 3, Macbeth worries over Banquo and she responds, “You must leave this” (p. 46). She does not want him doing anything to mess things up.
At the banquet, Lady Macbeth makes excuses for Macbeth’s weird behavior. She seems to be able to think on her feet, and be convincing.
Sit, worthy friends; my lord is often thus,(65)
And hath been from his youth. Pray you, keep seat.
The fit is momentary; upon a thought
He will again be well. (p. 51)
Eventually, though, she cracks. As the enotes character anaysis describes, "in contrast to Lady Macbeth's forceful disposition on the first three acts of the play, her actions in the last two acts are much less confident or ambitious" (enotes character analysis, Lady Macbeth).
By Act V she is sleep walking and seeing imaginary blood on her hands.
Out, damned spot! Out, I say! One–two—
why then ’tis time to do't. Hell is murky. Fie, my lord, fie! (p. 77)
Then she apparently kills herself, and is not seen again. Her breakdown and suicide are directly related to her feelings of guilt at her part in Duncan’s murder and the person her husband turned into after she spurred him on.
For more character analysis, read here: http://www.enotes.com/macbeth/lady-macbeth-character-analysis
Enotes. "Macbeth." Enotes.com. Enotes.com. Web. 15 May 2012. <http://www.enotes.com/macbeth/lady-macbeth-character-analysis>.
Shakespeare, William. "Macbeth." Enotes.com. Enotes.com. Web. 15 May 2012. <http://www.enotes.com/macbeth-text>.