The height of what we call the Page Banner (i.e. a banner created by uploading a thumbnail image to a page or blog list, event list, or product list, and editing the page description) is determined by padding. You can change it here (edit 180px and 155px to whatever you want):
padding: 180px 0 155px;
To make the content appear vertically centered with the transparent header, the top and bottom padding are not exactly the same, so you'll have to eyeball it a little.