Neptune is known to be the 4th largest planet in the solar system. This makes this almost 58 times bigger than Earth. This means that about 58 Earth-sized items can be placed inside Neptune. Just imagine how big that planet can be. It may have more space than earth but it does not have the qualities that make earth livable.
This is known as one of the ice giants. It contains a lot of ice but it also contains a lot of gases. This does not have any solid surface. If a spaceship would try to land, it will not be able to land properly. It will sink slowly into the liquid center of the planet.