different widths for blog and single post and pages

I can set the width in customizer > layout

However, I want single pages and post to be a different width.

I have tried setting layout to 1100
and used this
body.single .site.grid-container {
max-width: 800px;
}

But that does not change the single size

In fact, I've used this in the past, but it no longer works

body.blog .site.grid-container, body.archive .site.grid-container {
max-width: 1100px;
}
Got any ideas how i can get this done?

Thanks