The Golden Snitch, as Bowman called his invention, was a walnut-sized ball exactly the weight of a Snidget. Its silvery wings had rotational joints like the Snidget’s, enabling it to change direction with the lightning speed and precision of its living model. Unlike the Snidget, however, the Snitch had been bewitched to remain within the boundaries of the field.
Quidditch Through the Ages - pages 14-15 - Scholastic Books
Though there is no limit imposed on the height to which a player may rise during the game, he or she must not stray over the boundary lines of the pitch.
Quidditch Through the Ages - page 27 - Scholastic Books
I'll check to see if there's any more info and edit if so.