Change Page Post Width and Homepage Post Width

I've used the following CSS, which I found on this forum, to change the post width on my blog:

.single-post #main {
    max-width: 620px;
    margin-left: auto;
    margin-right: auto;
}

This is how it looks:

https://aussiehorror.com/nightmare-wax-goodbye-max-hello-marie/

I also want to change the post width on pages and the featured post on the homepage to 620px as per the above, but don't know what code to use. (I've tried doing everything through elements without any luck. Maybe there's some setting blocking the changes.)

Many thanks!