Should be 15.9gal in all of the BD/BG/BK Legacys and Legacy Outbacks. I think some of the early Imprezas had the smaller tank, but I've never seen a Legacy with the 10.2gal tank.
Unless you don't believe you have a functional "low fuel" light, just run it until you get that. Mine's pretty repeatable that it'll take 12-13gal when the light comes on. E is about 14gal for me.