No map is truly "big" enough for 300+ players.
We had this same problem in AW. The map is technically "big" enough, but the gameplay still has problems.
The root of the problem is the sheer number of people in the arena, and the fact that people naturally go to where the action is. With 300+ people in one arena, you are going to have these massive ganging furballs regardless of how large the map actually is. As the map gets bigger and bigger, you just end up having a mostly empty map with most the people in a few massive furballs localized over just a few bases.
This has all been done before, with the same results.
AW's map designers went so far in one map as to put a massive frame-rate killing mountain range in the middle of a map, to try to spread out the fights--even this drastic step failed. In fact this made the country-ganging even worse. They also tried reducing airfield ZONE limits--this too failed (people just took off from the next base over).
The best solution to these massive 40-50 plane rampant ganging furballs isn't a bigger map, but smaller maps in alternate arenas. Dividing the player base, while not really desirable, is something that will have to happen eventually.
AW found numbers greater than about 250 in one good-sized arena to be undesirable. AH seems about the same; with the normal AH map size I find the best fights when the arena has about 200-250 people. Over 250 or so and it starts to turn into a gangfest.
AW also used small (about 128 x 128 mile), 150-player limit maps for a long time. While this further divided the player base, it also kept the furballs fairly small and tended to balance out the country gangbanging somewhat. IMO these were the most "balanced" maps.
I think AH should limit the MA to no more than 250 people, and add "MA 2" to hold the overflow (basically they already have this except the name and numerical limits are different, and scoring doesn't seem to work right in the alternate MA). A good idea would be to make sure that different maps are always in rotation in these arenas. AW went so far as to have completely different plane sets in their various arena (with "europe", "pacific", and "mixed" arenas), but I don't feel such a drastic step is necessary in AH.
Just providing all the info I can. Make what you will out of it.
J_A_B