15x13 is fine. It's what Bomberman Online operates with and neatly houses 6 players. The only way I'd go larger is if it's more than 6 people playing.
I also like how some of the levels have 2 gaps between indestructible blocks. It takes more skill to dodge bomb blasts in that area. So if there was a larger area, they should make use of these open areas.
_________________ Hudson's Bomberman: 1 of the top 10 game franchises of all time.