iTheme 2 header re-size?

I have been using iTheme2 by Themify taken from the WP list, but I’m having trouble figuring out how to resize the header. Ive searched through the style.css editor and not found the relevant line. I found some iTheme2 docs and screen shots showing a ‘Themify Panel’ but couldn’t find the same editing features on my WP version. I contacted Themify and they said i should take it up with WP… so here i am.
I just want to change the size of a custom header. Im pretty new to this so give it to me in British. Ive searched the code but its not obvious and i don’t seem to have a resize option in the theme options. Help.

Thanks.