Mr Reid said he made "no apology" for sending more troops than previously expected.
"The size and structure of the task-force has been guided by a careful assessment of the likely tasks and threats it will face.
"What matters is that we put the right forces in to do the job and to do it safely and to do it well. And I make no apology for that if that has required more soldiers than some people initially envisaged," he told MPs.
He said it was not a "counter terrorism" mission but the additional support would help prevent Afghanistan from "falling back into the clutches of the Taleban". The operation would also help the country tackle its trade in opium - the raw material for heroin, he said.
Shadow defence secretary Liam Fox said his party would hold the government to account on the deployment.
1,100 already in Afghanistan - some to return home in the summer
1,000 - initial deployment to Rapid Reaction Force HQ
850 - (including engineers and marines) to prepare for main deployment
The initial deployment would be 1,000 troops to the Headquarters Group of the Allied Rapid Reaction Corps, with the main deployment of 3,300 heading to the south, including a Provincial Construction team.
Explaining the decision, Mr Reid said: "We cannot risk losing those achievements. We can't risk Afghanistan once again becoming a sanctuary for terrorists, we have seen where that leads be it in New York or here in London."
In May the UK takes control of Nato forces in Afghanistan, with soldiers due to oversee reconstruction efforts.
BBC defence correspondent Paul Wood said the deployment had been described as the most ambitious and challenging mission yet undertaken by Nato.
The Taleban, adopting suicide bombings as a means of attack, are very active in Helmand province, which is also a big opium-growing region.
Nato's International Security Assistance Force (ISAF) mission currently numbers about 9,200 troops. It is expected to increase the overall number to about 15,000.
Mr Reid said it was hoped other countries - including Australia, New Zealand and the Netherlands would also send troops to strengthen the Isaf.