How Can I Change the Header Size?

Hello,

I would like to make the headers (H1, H2, H3,...) much smaller. I tried looking in CSS stylesheets and the only one I found any references to headers was the Visual Editor Stylesheet however I think thats to the Atahualpa editor because any changes there did not change anything in my blog.

Prior to posting here I tried to figure this out myself, I also searched the forum here. I need help please where can I style the headers? If the answer is a php or css doc please tell me the name of the document.

Thanks,

Mike