(Standard disclaimer: I am not a lawyer, but based on the training I have had, I understand the following to be true.)
To be able to use lethal force, one must be in immediate fear of the otherwise unavoidable grave bodily injury or death of themself (or in some cases, another innocent person).
It is generally accepted that for that immediate fear to legitimately exist, the attacker must have:
the Ability to cause said injury or death,
the Opportunity to cause said injury or death, and
shown Intent to immediately cause said injury or death. (Often referred to as "Jeopardy," as a mnemonic device for police, who preform Administration Of Justice.)
If Ability, Opportunity, and Jeopardy all exist, or if any reasonable person in the victim's place would believe that they exist, the use of lethal force for self defence is most likely justified. (I say "most likely" because of the "otherwise unavoidable" bit above, but that's a subject for a whole 'nother thread.)
So, where does Disparity of Force come in?
It is one of the things that can fulfill the Ability requirement. Any weapon already counts as the Ability to cause lethal injury, so Disparity of Force only applies to unarmed attackers.
The situations outlined my mr_dove are a pretty good list of the things that can be considered Disparity of Force, with one exception and one that he missed.
The exception is the superior numbers situation. 2:1 may or may not cut it. Facing 3 or more unarmed attackers, you're pretty solidly into the realm of Disparity of Force with legal precident on your side, but facing only 2, you are in a very grey area. Not saying you couldn't use lethal force, but you definitely don't have an unquestionable green light, either. Also keep in mind that disparity can be removed as attackers are removed from the fight. Meaning that if you are attacked by three people, and you shoot two of them, you no longer have a Disparity of Force situation as you draw bead on the third guy.
The other situation is if you know, for a fact, at the time you are attacked, that your attacker is an expert in the deadly martial arts, and you are not, then Disparity of Force exists. (Actually, even if you are an expert too, you could probably argue that you were being attacked with lethal force, it just wouldn't be disparity). So if, for example, you are attacked by Jean Claude Van Damm, you would have a Disparity of Force situation. If you are attacked by Joe Schmoe, and it turns out after the fact that he has a black belt in Tai Kwon Leap, but you didn't know that when you shot him, then you can't argue Disparity of Force.
Also, as for a handicapped person having DoF versus an able bodied attacker, it is possible to become "handicapped" due to an injury during the altercation. So you could be attacked in a manner that would not justify lethal force initially, but would would once you become injured. But that goes the other way, too.
As for relative size of a man vs a woman, if rape is attempted, then lethal force if justified. Otherwise, it still very likely that DoF exists, based on averagy body size & strength, and cultural dispostioning. Even the 6'6" 300# woman being attacked by the 5'2" 125# man can have disparity of force if she has been so conditioned througout her life that women are inferior to men that she is psychologically incapable of fighting back physically.
Basically, it comes down to what the person being attacked reasonably believed to be the case when they were attacked, and being able to back that up in court.