@SophieZH: If you want additional CSS to make the content size fit to the screen width, add the CSS below to your site.
.single .is-layout-constrained > :where(:not(.alignleft):not(.alignright):not(.alignfull)) {
max-width: var(--wp--style--global--wide-size)!important;
}
.single .wp-block-post-comments {
max-width: var(--wp--style--global--content-size)!important;
}
Please navigate ‘Dashboard -> Appearance -> Editor -> Styles -> Additional CSS’ to add the code. Below is the screenshot for reference.

Kind Regards,
Sujapati