box size

Does this help: https://spacedonkey.de/2960/collapse-o-matic-grey-bar-style/

Thank you, but no –
I want the gray box to have a width that fills my entire page.

The link we shared shows the basic CSS required to create a box.
1. provide a trigclass
2. define the css for that trig class

width is a css attribute

If you are using a div element for the trigger, it will by default be a block element that takes up the entire width of the wrapping container.