Another point worth keeping in mind is that a really large group of units may require a more sophisticated system for handling orders than a small squad or blob does. I don't think having such huge groups of units will really work that well unless they are composed of distinct subcomponents which each do their particular functions intelligently.
The way I am envisioning this is that a large group of units is arranged into a hierarchical force organization, containing subdivisions. Such as having a few different armor groups, maybe fire groups, assault, anti-air, artillery, scout, and so on.
When the player selects the entire group and gives it an order, the entire group has to decide how to execute that order, which will depend on what the order demands. If it is a "Fire" order, such as a fire mission for indirect fire weapons, then only indirect fire weapons can fulfill that task, and there's no real need to bother units that lack that capability. Conversely, when ordering a direct assault, most likely soft assets like artillery and anti-air should not participate. Or when ordering a target area be scouted, it would make sense to send only a small scout detachment to the targeted location, and not move the entire army.
This is significantly more complex than the classical RTS system of box-selecting units and giving them all the same order, such as to move to a target location. If that group has 1,000 units then I think it would be a far more common case where you actually want the group to delegate intelligently, rather than always give the same orders to everyone.
To implement this, perhaps the force hierarchy could have a list of orders which are active, and the group will attempt to fulfill all of those orders. This may include simultaneous orders of several types, which would be impossible in an RTS like Starcraft.
For example, you could order a group to attack four different locations. Under classical RTS controls, the entire group would receive four sequential orders, each overriding the one before it, and the entire group does an attack-move to the fourth location.
If instead the group remembered the four locations, it might split its direct attack capable units into four attack forces, and have each group advance on one of the objective points. The entire group is still logically the same entity, but its disposition changes based on its orders.
The way this system could look visually in the UI is having multiple icons/flags out on the map, possibly with indicator lines connecting the group to its currently active objectives.
A group should clearly indicate where it considers itself to be located (its own position), possibly with some kind of icon or platter beneath the units. And it will probably need a clear indicator of its "destination" if it is moving, indicating a traditional RTS order to relocate everything in the group to the target point. By dragging around the destination you could give move orders without even needing to box select at all.
But on top of these indicators, there might be various other icons, the player can create by issuing the group specific orders, like fire artillery at positions X, Y, and Z, and post a sentry at A, B, and C, and post an anti-air battery at positions J and K, and so on. Potentially this could even be used for interactions between groups, such as a land group requesting an air strike which an air force group fulfills.
The point is that when the player issues an order like this, it would be added to the group's list of active orders rather than immediately overwriting whatever the group was doing before. This would allow a very large group with many units in it to intelligently delegate some appropriate units to accomplish a specific task, rather than always require that the player use the entire group.