Hi.
override.css
What is this file? In Helix should be used custom.css file. If not exist, create it in
/templates/YOUR_HELIX_TEMPLATE_NAME/css/ folder
reduce the size of the container
The width of the container is not a subject to solve your problem. First you need to fix the problem with empty side bar, then you can configure the width of the container as you need.
Here is your page layout looks like

You should adjust Component area width to 100% of the container width. After that, the right side bar will move below the Component area and will remain not noticeable, as it is empty.
This code does just that. And this cannot but work if you do everything right.
@media (min-width: 992px) {
.fix-empty-right-position #sp-component {
width: 100%;
}
}
Then you can adjust container width as you like.
Example (select one of the options)
/* For all tipes of pages (it's affect whole the site) */
#sp-main-body .container {
max-width: 1200px !important;
}
/* For Category Blog page only */
.view-category #sp-main-body .container {
max-width: 1200px !important;
}
/* For single Article page only */
.view-article #sp-main-body .container {
max-width: 1200px !important;
}
/* For both - Category Blog and Article page */
.com_content #sp-main-body .container {
max-width: 1200px !important;
}
/* For pages with your specific class name */
.fix-empty-right-position #sp-main-body .container {
max-width: 1200px !important;
}
If it does not work for you, open the site for public access so that I can see and check your actions. I am not an employee of support.