The map room is indeed massive, and it can be very troublesome to place. If you're not able to place it in an open space because you're in a dangerous area or on a PvP server, the best method is to build an 8x8 room with foundations, a single row of walls, and then place the map room down from a slight elevation. Don't try to add roof pieces or extra walls beforehand, as this can stop the map room from being able to be placed. A map room will also fit perfectly inside a 7x7 room with walls two tiles high, the placement is very tight and requires practice, but it can be done