A few previous major oil spill disasters for comparison ...

The Louisiana oil spill's size is yet undertermined but here are some others for comparison.
(From http://en.wikipedia.org/wiki/Oil_spill )

The largest ever:
Gulf War oil spill -- January 21, 1991 -- 9.5-11 million barrels
Maximum size of 101 by 42 miles and was 5 inches thick.
About half the oil evaporated, a million barrels were recovered and 2 to 3 million barrels washed ashore, mainly in Saudi Arabia.
A study sponsord by UNESCO and several oil producing nations said it did little long-term damage.
Iraqi forces dumped oil from tankers to prevent US marines from landing.

Largest oil well blowout:
Ixtoc I oil well -- June 3, 1979 -- 3.3-3.5 million barrels
Gulf of Mexico 110km NW of Ciudad del Carmen in Campeche
11,800 feet below the seafloor.
10,000-30,000 barrels per day released
Lasted about 8 months

Largest American oil spill:
Exxon Valdez -- March 24, 1989 -- 250,000 barrels
Prince William Sound, Alaska
Many overall reduction in animal populations over the years.
Still feeling the effects today.
May take 30 years to completely recover.

American oil well blowout:
Santa Barbara oil spill -- January 28, 1969 -- 80,000-100,000 barrels
Six miles offshore
Lasted 10 days

Today's oil gusher:
Deepwater Horizon -- April 20 2010 -- 5000-25000 barrels per day
5000 feet below surface
50 miles off Louisiana coast
Duration unknown.
Maximum amount released unknown.

From the references I found, it seeems this spill will be by far the largest ever near
a major populated coast.

What a disaster.