A lot of afcads that have been reworked by other people and now are larger than 2000K. I would suggest that you might want to put the answer in the frequently asked questions.
We'll likely put this in the manual, because right now this feature has been discussed on the forum only.
However, I still believe a smart scenery designer would separate the airport data from things that don't really belong to it, such as textures for example, which is the reason why AFCADs grows so large. No commercial scenery developer does it, because they all have enough experience to know that it's best to keep the airport information required by the ATC/AIs separate from the larger visual stuff.
And, of course, there's a reason why we chose 2.0MB as the highest threshold for an airport file: the largest AFCAD in the default scenery is 1.4MB and they usually cover several airports. Payware developers usually have a single AFCAD for each airport, and they very rarely surpass 500K.
So, having 2.0MB as default, is a sensible choice, which doesn't force everybody to have their cache rebuild time slowed down, just to accomodate for those large AFCADs that includes stuff that is probably best being kept separate.
But yes, we'll add it to the manual nevertheless.