I received this and thought it was interesting:

Largest Hotels in the United States
Ranked by Number of Hotel Rooms as of June 30, 2004

Rank Property Name Location Number of Rooms
1 MGM Grand Hotel and Casino Las Vegas 5,034
2 Luxor Hotel and Casino Las Vegas4,408
3 Mandalay Bay Resort and Casino * Las Vegas 4,337
4 Venetian Resort - Hotel - Casino Las Vegas 4,049
5 Excalibur Hotel and Casino Las Vegas3,991
6 Circus Circus Hotel, Casino and Theme Park Las Vegas 3,770
7 Flamingo Las Vegas Las Vegas 3,565
8 Las Vegas Hilton Las Vegas 3,174
9 Mirage, The Las Vegas 3,044
10 Bellagio Las Vegas 3,005
11 Monte Carlo Resort and Casino Las Vegas 3,002
12 Paris Las Vegas Las Vegas 2,916
13 TI (Treasure Island)Las Vegas 2,885
14 Opryland USA Nashville 2,881
15 Disney's Pop Century Resort Orlando 2,880
16 Hilton Hawaiian Village Honolulu 2,860
17 Bally's Las Vegas Las Vegas 2,814
18 Imperial Palace Hotel and Casino Las Vegas 2,700
19 Harrah's Las Vegas Hotel and Casino Las Vegas 2,575
20 Aladdin Resort and Casino Las Vegas2,567

* Mandalay Bay Resort and Casino also includes THEhotel at Mandalay Bay

Las Vegas has:
10 of the 10 largest hotels in the United States
13 of the 15 largest hotels in the United States
17 of the 20 largest hotels in the United States

With the addition of Orlando's recently opened Disney's Pop Century Resort (2,880 rooms), the Rio was bumped from the Top 20 list. Also, be advised the Hilton Hawaiian Village listing reflects only their hotel room inventory and does NOT include 390 time-share units within their complex.

Source: Las Vegas Convention and Visitor Authority