my friends play on a 6'x6' board, they have more than enought room for 4 fleet games, I dont know very much about BFG but i made the board for them ( and used it as art course work)
Just spray an appropriate size board black, use white spray from a long distance to creat stars, and paint a mix of reds and yellows on to some car and use it to make a circle for a planet
simple and effetive result!