The biggest crowd they've ever played to was at Live 8 in 2005. Their biggest headlining show was probably one of their many European festival appearances...I know one of their Rock in Rio performances in Lisbon had between 90,000 and 100,000 fans in attendance.
Sao Paulo 2004 was the biggest, I'm pretty sure. Other big shows were Live 8, Shanghai 2007 I believe, and a few others. I'm not sure which show had the MOST people in attendance, I wish someone could find that out.