just by tidying up the UI (not even resizing) you could easily fit 2-4 more units into it.
if you increase the size of the middle group field just slightly, you can fit in another 4. dont forget that the early wc3 was played on 4:3 monitors, therefore the UI covered the whole bottom fourth of the screen. the empty space on the left and right just came with widescreen monitors. we have lots of room to use at our disposal.
and theres another thing to it: just because you could theoretically select more than 12 units, it doesnt mean you have to. i mean, most of the time you will have 4 to 8 units in a group, and sometimes you will have more units in a group. it might be useful to split 16 ghouls into two groups, but it also might be useful to have 16 gargoyles in one. it should be player choice, not game limitation.
on the matter with making single unit selection via the group window like healing - thats really not a thing you should do when playing competitively, as it takes away your focus from the battlefield. you should eyeball hurt units on the field and move units in mouse selected small packs, not in groups.
using the group window is good for new players, as they lack the overview and stress management to do battlefield only micro, and there it helps to have bigger groups, as you can overview more units at once. to get better though, players will need to learn how to do “no UI” fighting and controlling. but its ok to start with the group window - wc3 is really hard to get into, and we should offer anything to help new players to get into it. and right off the bat i can guarantee, that a 12 unit selection limitation, thats probably only half the army of a low level game (building army, creeping, less fights), is very discouraging for beginners.
let them have fun first, before you smack them with the hard reality of competitive multiplayer, “git good” harassing and “uninstall” flames.