We actually haven't set any size limit for personal use ships in the Residential Zones yet, just don't build something so colossal that it will cause lag and/or crash the server.
Inside the Residential Zone there are almost no limitations. Aside from several locations containing races/obstacle courses/mazes/arenas and other minigames, you may purchase your own planet, station, or star system. You may then rent/sell segments of your planet, rooms in your station, or sectors of your star system to other players if you wish. Raiding is allowed, so it is recommended to set up a homebase asap. Excessive griefing and spawn killing is discouraged (will result in kicks), and destruction or tampering of Admin Owned property is forbidden.