Install fire bug for firefox and then right click header and then inspect element. Change the styles appearing and see the changes. If you find out, then make the changes to css from admin area. I am using ethos theme and I changed header size like this
.ow_console {
height: 50px !important;
}
.ow_header .ow_logo {
height: 50px !important;
width: 150px !important;
padding-bottom: 0 !important;
padding-top: 0 !important;
margin-left: -20px !important;
}
.ow_header .ow_logo h2 {
height: 50px !important;
margin: 0 !important;
padding-top: 0 !important;
}
.ow_header .ow_logo h2 a {
height: 50px !important;
width: 150px !important;
}