Hi Jamie,
The header_background is there to provide a 40px height background. and it will enlarge when users use the banner-wide widget position. but if you wish to use the header_background itself you can use the following css to control the height .
div#header-caption {
height: 260px;
}
You can insert your desired height in the height property.
Note: you need to use child theme or custom css plugin to insert the codes as this helps preventing the data loss during the updates of the parent theme.
Thanks
