how to have a different width for post and archives?

Hi there,

We are working to add the container width option into Layout Elements :)

For now, I believe the best way to handle your case would be to let the customizer option to handle the widths of the layout with sidebars.

Then we can use this CSS to set the content container width when there isn't a sidebar:

body.no-sidebar .site.grid-container {
    max-width: 600px;
}

Adding CSS: https://docs.generatepress.com/article/adding-css/

Let me know if this helps :)

while it works, this creates problem with the header, that is no more aligned with the content

You can add another rule to give the header some padding:

body.no-sidebar .inside-header {
    padding-left: 47px;
    box-sizing: border-box;
}

this a bit too much code.

could I instead add an element with content/sidebar and width contained that displays to archives or specific pages and add a css rule like

.body-sidebar-content .site-grid-container {
width:900px

}

I tried but it didn't work

Not quite sure what you mean but what about changing the width of the entire container including the header on pages no sidebar pages?

body.no-sidebar .grid-container {
    max-width: 600px;
}