Your theme should be including certain classes within the <body>
tag that you can use to target specific pages (and posts, categories, tags etc..).
For example, if your page has an ID # of 5, you can target that page only with some basic CSS code, like so:
.page-id-5 .container {
width: 1200px;
}
You can add custom CSS code using a Child Theme style.css or a Custom CSS plugin.
You can easily view the ID # of any of your WordPress elements with the handy Reveal ID’s plugin.
If you need further help, just add a link to your site and I’ll show you wihch class you can modify.
Hope this helps.