I'm not really very good at such things and I claim no true expertise, but I have had a little experience looking at crowds and figuring out how big they are, and I can tell you this: this was a BIG crowd.
The video gives only a flavor of the rally; I was too far from the podium to either see or hear the speakers very well (although I was glad to see my Senator, Dick Durbin, showing solidarity with the crowd) so don't look for speeches. But I spoke to several people to ask them why they were there. None of them were paid by wealthy ideologues to attend. They wanted to be there. Take a look and a listen.