ORIGINAL: Mogami
Hi, Midway Naval Air station was built pre-war. Midway did operate loaded B-17's at normal range (so it has to be a size 6)
During the Battle of Midway it was not limited by it's size but by the fact it is outside the transfer range of most aircraft.
Hi Mog,
I would offer a slight correction to your statement -- Midway did operate B-17s at normal range, but the one atoll with an airfield (Eastern Island) was limited by its size and probably could not have handled more than the roughly 100 land-based aircraft (plus 32 Catalinas) that were based there during the Battle of Midway. The following image shows how the island looked during WWII and how the twin-runway airfield basically filled the entire island:

It's hard to imagine 300 airplanes (airbase 6 maximum) being based on that island!
A game of this scope has to require compromises. The game formula for airfields (capacity = airfield size x 50) works well for land bases, less well for small islands. In the case of Midway the designer/player has a choice . . . an airbase size of 6 will accurately represent the ability of B-17s to use the island, but will allow an ahistorically large number of aircraft (300) to operate from the island . . . an airbase size of 3 will accurately limit the total number of aircraft on the island, but will ahistorically restrict B-17 and medium bomber operations. Choose your poison.