Logo Size Mainstream theme

I’m trying to resize the logo. I can’t seem to find the code in the style.css in the theme. I found this

#header #logo {

float: left;

width: 470px;
}

and have added height and width but it does not effect the logo size. The problem is the logo is the same size as the orignal one, but once uploaded the code is increasing the size of it, so that it looks blurry. Thanks.