When people say DoubleClick, they could be referring to either DoubleClick Select (formerly known as Doubleclick Network) or Doubleclick Audience (formerly known as Sonar).
Doubleclick Select will not let you into their network unless you are a recognized national brand with a plan. They used to say you need to have a minimum traffic level, and that used to be 1,000,000, but that number was never used, and in the days when traffic levels were the measure used, the number they were using was probably more like 10 million/month.
Doubleclick Audience has much looser standards. Depending on who you talk to, they're looking for 100,000 or maybe 250,000 per month sites. They have recently been letting in a lot of publishers, many more than in past months. I attribute this to their $0.25 CPM default serving fee -- they don't have to be selective anymore, because the more publishers they let in, that's more revenue they can collect from default fees if they can't sell the space. It's a win-win situation for them, they've taken all the risk and put it on the backs of the publishers.
Ralph