The best thing I can think of is the Alexa toolbar. You can find out the total number of page views for the entire site and other bits of site-wide information.

The best thing I can think of is the Alexa toolbar. You can find out the total number of page views for the entire site and other bits of site-wide information

I believe it only counts vistors that have the Alexa toolbar installed.

Alexa can give you approximate amount of visitors. But it is only public tool to imagine the site attendance

Ask the site owner for traffic details saying that you are doing banner advertisement for xyz-big-site.com and want to place one banner in his site, he will exaggerate the traffic data but you get the estimate