container size

ok i believe what i want to accomplish is to change the size of this:
div.menu-back
it is currently 1282×241

i have changed all the padding that i can on the items around it and this is the only thing that is holding me up from shrinking this top section height on my page.

it seems that when i want to make changes to base.css i have to do it through the Appearance section by going to Edit CSS and typing in exactly what I want. I can’t just go into base.css and directly change the base.css file.

Can anyone offer some input on the code I could insert on this page to change the size of this section?
Thank you as always!
Jenn