You can just target a single page section by wrapping the whole content within a div with a certain id. By default a page section is full width which means unless you wrap something within a [segment] shortcode, the content occupies the entire browser width. So, instead of starting the page content with [segment] shortcode, you can start with a regular div and then have custom CSS like what you posted above which targets only that div like this
#my-custom-wide-div {
padding-left: 20px;
padding-right: 10px;
max-width: 1620px;
margin: 0 auto;
}
while you leave the .segment-content styling undisturbed.
There is one other alternative too which may be easier. Just use a [segment id=”my-wide-segment”] to wrap the content and then set the custom css to be –
#my-wide-segment .segment-content {
max-width: 1620px; /* Override the default max-width of 1140px */
}