How to make header larger on Marketpress/Gridmarket

HI I would just like to be able to change the size of my header For now I'm trying to keep the same width - 980 - but make it 150 tall.
I have added this code to custom css:
#branding h1 a {
height: 150px !important;
width: 980px !important;
display: block !important;
text-indent: -9999em;
#header-wrapper {

it seems to make the space above the header larger, but when I try to upload a 980 x 150 header image I am still forced to crop before submitting. Can you offer any guidance. Thanks!