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I noticed today that in the pilot of Star Trek: The Next Generation, Data shows Riker one of the four "walls" of the Holodeck by throwing a rock at it. Thus, it seems logical that the Holodeck would be bounded by these walls, making it the size of the actual room only. Yet, throughout the series ("The Big Goodbye", "Booby Trap", "Emergence", "Ship in a Bottle", and so on...) the Holodeck appears to extend regardless of the room and (more importantly) the characters in the Holodeck are able to move around in the environment with no limitations. Even when characters move in opposite directions, they still are not limited by the actual Holodeck room. Why is this?
tl;dr Why are there seemingly no limitations on the Holodeck with regards to area?