I was on Facebook and came across a page called UltraMegaHostels:
A brand new hostel for boys, with a 1600 rooms, both single-bed and double-bed, With eight elevators installed, good precautions regarding hazards, emergency escape stairs are installed. LAN facilities to every room, with 2 ports for the double bed rooms
I guess "Mega" wasn't a big enough word to describe the 1,600 rooms and eight elevators. :)
It isn't a hostel for travelers, but check out photos here and here.
Is the largest travelers hostel in the world still Danhostel? (1,020 beds)