When I started 0.9.1a, I began with the default of 300. Once I reached multi-capital fights, 300 was too small because every fight was a series of multi-round fighting game-like 3v3 duels, reminiscent of Mortal Kombat endurance matches, which I thought was nonsense, and raised map size to the max of 500. Even 500 is barely big enough.
300 is too small for the current release. It would be fine for pre-0.8a or possibly pre-0.9a releases. Endgame fleets of prior releases were the size of a modern 200k bounty.
With my old computer, 500 is fine for cruiser and battleship fights, but not for Drover-and-Spark spam.