This is the description of the dungeons leading to the Slytherin common room in Chamber of Secrets:
"The Slytherins always come up to breakfast from over there," said Ron, nodding at the entrance to the Dungeons. The words had barely left his mouth when a girl with long curly hair emerged from the entrance.
"Excuse me," said Ron, hurrying up to her, "we've forgotten the way to our common room."
"I beg your pardon?" said the girl stiffly. "Our common room? I'm a Ravenclaw."
She walked away, looking suspiciously back at them.
Harry and Ron hurried down the stone steps into the darkness, their footsteps echoing particularly loudly as Crabbe's and Goyle's huge feet hit the floor, feeling that this wasn't going to be as easy as they had hoped.
The labyrinthine passages were deserted. They walked deeper and deeper under the school, constantly checking their watches to see how much time they had left. After a quarter of an hour, just when they were getting desperate, they heard a sudden movement ahead.
From the last paragraph it is clear that the Dungeons are simply massive. While it might have been safer to keep the students in the Great Hall, Dumbledore may felt that there was little chance the Slytherins would actually run into the troll if they just went straight to their common room.