Themify 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 a custom 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.