Single posts narrow content container

Greetings,

I would really like to have a different container width for some posts. Right now the costum CSS in Customizer has this snigle posts narrow content container:
/* Single posts narrow content container */
.one-container .entry-content, .comments-area, footer.entry-meta {
max-width: 725px;
margin-left: auto;
margin-right: auto;
}

my site is: https://thegreatestsong.com

I use elementor page builder with generate press as my theme. Forgive my ignorance, but could you tell me how to modify this container for other posts. Many thanks for your help.