Yellow Anacondas usually get to about 10 foot for males, 15 foot for females (I think)
You might wanna look into getting him a big half-land-half-water enclosure, like one of those big white plastic ones with the sliding front glass. You can NOT free-roam a Snake. They aren't like Monitors or Iguanas or Tegus. They can just fit through too small of space. And then you have a potentially 15 foot Snake in your neighborhood.