>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**.<br><br>
>*Quidditch Through the Ages* - pages 14-15 - Scholastic Books<br><br>
>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.<br><br>
>*Quidditch Through the Ages* - page 27 - Scholastic Books

I'll check to see if there's any more info and edit if so.