I think you're asking for an answer that doesn't exist. The history of RPGs is the history of gamer interest moving from larger scale to smaller and the systems they developed to model those interactions at an entertaining and manageable level.
Here's what I mean:
Wargames focus on mass actions. Each piece in a wargame could represent dozens or hundreds of individual combatants, or large vehicles manned by the same. Game mechanics and interactions answered questions about those units:
- Does the unit have the ability to receive and follow orders?
- Does the unit have the wherewithal to inflict damage on the enemy?
Gamers interested in more detail reduced the scale of the combats. Pieces represented squads, so the game answered questions about them:
- How many members of the squad are still functional?
- What can the squad see and fire upon?
Gamers interested more in the members of the squads than the squads themselves formed the basis of the first RPG designers and players. They created mechanics that answered questions about individuals:
- What injuries has this individual sustained?
- How much damage can this individual inflict?
- Will this individual continue to operate effectively or break?
- How much equipment is this individual carrying?
So these different games systems evolved to address questions that were interesting and manageable at the level of detail that the gamers were concerned with. Those different systems evolved precisely because the questions the system is asked to answer are different and therefore the mechanics of the system to answer those questions must be different.
For instance, I think it's pretty obvious that if you have a good simulationist system for individuals in combat that you could scale it up to simulate combats between thousands of individuals. I think it's similarly pretty obvious that you couldn't run that combat manually at the table at a pace that would allow you to complete even a single round of action in the amount of time most people have to play. It's also possible (and even likely) that a bottom-up simulation like that would require additional layers of rules to answer the questions that you would be interested in at that larger scale.
This is why systems (GURPS included, but I think you'll find that Savage Worlds and others have similar systems available) have been created for mass combats within RPGs where you are interested in questions on both extremes of the scale. Questions like:
- Who won?
- What did the PCs do during the battle?
- How did the actions of the PCs impact the course of the battle?
- What is the condition of each side's forces after the battle?
- Were any PCs injured? Killed?
I guess this is a long way of saying that the questions you want the system to answer for each of the scales you've provided are different and that no single set of mechanics can provide the answers to all of those questions.
The comments are clamoring for me to address the "elephant in the room" - the apparent existence of such a system already. I tried to continue this conversation in the comments, but my reply wouldn't fit.
@SevenSidedDie - Are you talking about the part of this answer where it states, "...mechanics have been designed to slide easily from personal to vehicle/unit scale..."? Because it doesn't cover the range he asks for and so is not an "elephant". Many RPGs can handle vehicular combat - and handle it exactly like regular combat with bigger guns and faster movements, because you're talking about combat between individuals - individual vehicles. And many handle small-unit actions by iterating the individual combat rules as I mentioned above. I stand by my assertion that the essentials of each are different enough that when distilled, you find yourself with different systems. Unless you go to a completely abstract system, where combat itself is considered just another skill contest, so two fighters would contest using melee skills, two squad leaders using tactics, and two generals using strategy or something. That didn't seem to be the spirit in which the question was asked, though.