How to change the width of my custom image header (reasonpad.com)..pls help

I am finding difficult to change the width of my custom image header (reasonpad.com) . It says pls upload an image of 576×131 if i click on “custom image header” on my dashboard. I tried changing 576 to 876 in style.css but with no effect. I want my logo image header on the left to cover up most of the hear part till “subscribe button” of rss. but its not taking anything more than 576.

Pls help.