"We'll be doubling our size if all of our plans go through and if we receive the money from Congress in 2011 and then again in 2012," James Jeffrey, the US ambassador in Iraq, said.
He said the staff would increase "from 8,000 plus personnel that we have now to roughly double that by 2012," adding that US forces would make up only a very small part of that number.
"This will be an extraordinarily large embassy with many different functions. Some we took over from USFI (United States Forces in Iraq) and some of them continuation of the work we are doing now."
Mr Jeffrey said that US military advisers and trainers would stay or be added to support the Iraqi military with US-made equipment such as M1A1 tanks and other weaponry. He said the added personnel would not include combat troops.
Fewer than 50,000 US troops are currently in Iraq, down from a peak of more than 170,000 and ahead of the planned full withdrawal in late 2011.
Jeffrey and Lieutenant General Lloyd Austin, the commander of US military forces in Iraq, told members of the Armed Services Committee in February that the embassy would be well protected after the withdrawal.
A private security force some 5,500 strong will protect the large US diplomatic presence in Iraq, Jeffrey told the lawmakers.
He and Austin said they were confident that the force was adequate, and that Iraq will remain stable once US troops have departed.
They said that in 2012, the American presence in Iraq will consist of up to 20,000 civilians at sites that include two embassy branches, two consulates, and three police training centres.
The figure includes armed private security personnel, support staff and diplomats.
Currently there are 2,700 armed security contractors in Iraq, Jeffrey told the senators.