Problem is without what @Shplendyt suggests, no matter how big one makes a map it will still feel small where it matters - which is the size of area where legitimate player interaction occurs.
Vietnam in ARB and ASB is the same size - 128 x128 km. By every measure of your request, it’s a pretty big map.
However, in ARB it does not feel that way. Is it because when I play ARB, it’s in jets while I do ASB in props? Maybe. But I think much more impactful is the fact that if you look closely in ARB - Vietnam’s ground targets are spread across some 3 gridsquares wide and the bases too are spread within that tight column, maybe reaching 4 squares across.
To add to this, airfields spawn within this 3-4 square wide column of relevant objectives and the shortest path from AF to AF coincides with the objectives so despite having 128 km width to work with, you narrow it down to just 3x16 (48) to 4x16 (64).
This seriously impacts player density and leads to furball formation.
If you try to be smart and use the entirety of the map and say, try to fly behind the enemy team to flank? Well - by the time you arrive, even if you had only taken a mere 3 minute long detour - the match is already over because the average player takes the most straightforward and intuitive solution to the problem presented.
What’s the solution?
Multiple airfields that are spread equidistant across the map with forced spawn distribution (to prevent everyone spawning in the middle) that respects squads and tries to balance aircraft types. This could be 3 airfields at jet tiers and 4 at prop tiers.
However, even with forced spawning you’ll still have the issue that players will just fly to the middle and it devolves back into a furball again and due to this, you’re effectively punishing those who need to fly longer.
As such, we need further development in introducing pivotal, game-changing and provitable objectives that reward first dealing with the flanks before joining the center of the map again.
In prop tier sim, we kind of have this with A.I bomber/attacker/survey intercepts giving good rewards alongside the “Air superiority point”, spreading out even more populated lobbies and this is before considering that ground battles appear in multiple grid squares (often disconnected) and minibases being scattered along the entire width.
It’s not perfect, and once you reach jet tiers the a.i gets broken flight models leading to supersonic sabres flying like dolphins (Community Bug Reporting System), outrageous damage models (Community Bug Reporting System) and not to be forgotten - after korean jet tiers they award miniscule rewards making them not worth doing. Ground targets also suffer from BR differences causing certain ECn brackets to get horrible rewards (For example you can encounter LekPz in 4.7-5.7 lobbies, and you’re still bombing LekPz in 9.3-10.3 lobbies)
But it does make a difference at prop brackets in diluting the player density, making even 64x64 km maps feel bigger than ARB’s copy-pasted EC maps.