So, I found a solution for this, in case anyone wants to do something similar.
First, I set the max content size in Lay Options to 800 px. This is for screen sizes up to 1280px.
Then, using the base code from the site I added the following into Custom CSS & HTML under Custom <head> content:
<style>
@media only screen and (min-width: 1921px) {
.row-inner{
margin-left:auto;margin-right:auto;max-width:1300px!important;
}
}
@media only screen and (min-width: 1281px) and (max-width: 1920px) {
.row-inner{
margin-left:auto;margin-right:auto;max-width:1000px!important;
}
}
</style>