As Primary Nears, Obama Takes His Family to Voters - From The Road: "The Obama family “picnic” drew a crowd of 4,000 to Headwaters Park, where his daughters took the stage for the first time since campaigning with their father. After introducing herself, Sasha said, “Vote for daddy!”"
I've seen crowd estimates all over the place, from 2,500 to 5,000. I'd say 4,000 is close to what I estimated while there. It was a LOT of people.