Airline size can be measured various ways. But almost any way you slice it, a combination of Delta and Northwest airlines would eclipse the current "world's biggest" title-holder. Of course, that could change if announcement of a deal prods and other big carriers into their own merger talks.

Here's how the world's carrier's stack up in key categories:

PASSENGERS
Measured by enplanements — the sheer number of people who get on planes
Delta+Northwest, 129.5 million
1. American, 99.8 million
2. Southwest, 96.3 million
3. Delta, 73.6 million
4. United, 69.3 million
5. Northwest, 55.9 million
6. Lufthansa, 51.2 million
7. Air France, 49.4 million
8. All Nippon, 49.2 million
9. Japan Air Lines, 48.9 million
10. China Southern, 48.5 million

TRAFFIC
Ranked by revenue passenger-kilometers — one passenger flown one mile. (This is the yardstick most widely used by financial analysts, because it measures all carriers on the same basis. For instance, a carrier built around short routes will rank high on passengers carried but may have far fewer RPKs than one with a lot of long-distance routes.)
Delta+Northwest, 275.80 billion
1. American, 224.33 billion
2. United, 188.68 billion
3. Delta, 158.95 billion
4. Air France, 123.46 billlion
5. Continental, 122.71 billion
6. Northwest, 116.85 billion
7. British Airways, 114.90 billion
8. Lufthansa, 114.67 billion
9. Southwest, 108.94 billion
10. Japan Air Lines, 89.31 billion

METAL
Planes in the fleet.
Delta+Northwest, 975
1. American, 655
2. United, 578
3. Lufthansa, 526
4. Southwest, 521
5. Northwest, 515
6. Delta, 460
7. SkyWest, 436
8. AirFrance/KLM, 400
9. Continental, 367
10. US Airways, 357

WORKERS
Full-time equivalent employees. Only figures for U.S. carriers were available.
Delta+Northwest, 78,300
1. American, 73,300
2. United, 52,400
3. Delta, 48,700
4. Continental, 36,100
5. Southwest, 34,100
6. US Airways, 32,800
7. Northwest, 29,600
8. SkyWest, 10,100
9. JetBlue, 10,100
10. Alaska, 9,900

SALES
Annual revenue, in U.S. dollars
1. Air France-KLM, $34.8 billion
2. Lufthansa, $33.7 billion
Delta+Northwest, $31.7 billion
3. American, $22.9 billion
4. United, $20.1 billion
5. Delta, $19.2 billion
6. British Airways, $17.3 billion
7. Continental, $13.8 billion
8. Northwest, $12.5 billion
9. US Airways, $11.7 billion
10. Southwest, $9.9 billion

Sources: International Air Transport Association, Bureau of Transportation Statistics, Reuters industry profiles

Note: Passenger and traffic figures are for 2006. Fleet, sales and employment figures are for 2007.