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.

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. 

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

Citations:

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>.

What is Lady Macbeth's appearance?

We know a lot more about Lady Macbeth's personality than we do about her physical appearance. But the two are closely linked. After all, if Lady Macbeth is to pull off her murder plot against Duncan, she needs to look the part and come across as a sweet, gentle noblewoman. One can safely infer from Duncan's complete obliviousness to what Lady Macbeth has in store for him that there's nothing about her physical appearance that raises suspicion.

Macbeth is set in an age where character judgments were often made on the basis of how people looked. Among other things, this meant that the disabled were subjected to widespread suspicion and prejudice, as their disabilities were adduced as evidence of wickedness. That Lady Macbeth elicits no such suspicion should at least assure us that she conforms to the prevailing standards at the time of what is considered "normal" regarding one's physical appearance.

As far as her specific physical features like hair or eye color, height, body type, etc., we really are provided with no information.  However, in order to fool everyone into thinking that she is the loyal and "honored hostess" that Duncan believes her to be, she must at least have a friendly and pleasing appearance (1.6.13).  When Duncan arrives at the Macbeths' castle after Macbeth does, he implies that Macbeth beat Duncan and his retinue because he was so anxious to get home to his wife, and this seems like a compliment to her both as a good wife and a beautiful one.   Further, he refers to her as "fair" when he expresses gratitude for her hospitality, and this is typically a synonym for attractive or lovely, but it can also be used to describe someone who is blond or pale.  Therefore, he could simply be complimenting her beauty or he might be identifying her complexion and/or hair color as well. 

In Shakespeare's Macbeth, what is Lady Macbeth's mental state?

In Shakespeare's tragedy of Macbeth, there is a complex shifing of things seen or imagined brought about by the voracious ambition for power by the doppelgangers, Macbeth and Lady Macbeth.  For, in their minds, there is much that is seemingly fair while it is foul. Thus, the play is truly a tragedy of the imagination.  Like her husband, Lady Macbeth suffers from the knowledge of her evil deeds, but this knowledge is delayed, whereas in Macbeth it is immediate. In addition, the reader is privy to more of the workings of Macbeth's mind, but because Lady Macbeth dies earlier in the play, the reader learns less of her motivation and nature.

Nevertheless, there are several aspects of her psyche that the reader can perceive:

• At first, her lust for power seems more forceful than her husband's as she summons the spirits to unsex her, seeking masculine brutality that will not waiver from its aim. Her unnatural absence of any feminine qualities is frightening as she calls upon the spirits,

unsex me here
And fill me, from the crown to the toe, top-full
Of direst cruelty! Make thick my blood,
Stop up the access and passage to remorse,(1.5.42-45)

• She is more treacherous than Macbeth.  For, when Macbeth has misgivings about killing Duncan, who has honored him, she berates her husband,

Art thou afeard
To be the same in thine own act and valor
As thou art in desire? Wouldst thou have that
Which thou esteem'st the ornament of life
And live a coward in thine own esteem,
Letting “I dare not” wait upon “I would”
Like the poor cat i’ the adage? (1.43-49)

• Her bravado is, nevertheless, rather superficial as it is motivated, ironically, by her love for Macbeth and her desire for his rise to power.
• She is humanized by her mental decline as her conscience tortures her.  In fact, there is much pathos in Scene 1 of Act V in which the insane Lady Macbeth yet tries to protect her husband,

Wash your hands, put on your nightgown;
look not so pale. I tell you yet again, Banquo's buried; he
cannot come out on's grave. (5.1.56-58)

• Further, Lady Macbeth is reduced to insanity and her spirit of determined cruelty and treachery disintegrates under her guilt and she descends into what critics have termed "a distinctly feminine Hell" as without support from her husband, there is no one to whom she can unburden herself:

Here's the smell of the blood still.  All the perfumes of Arabia will not sweeten this little hand.  Oh, oh, oh! (5.1.45-46)

Clearly, then, Lady Macbeth's imagination takes her down dangerous and complicated avenues that end in isolation, guilt, and insanity.