Perhaps you’re trying out different themes? The site that you linked to is currently running Twenty Fourteen, not Twenty Thirteen, so the CSS that you would use will be slightly different. For Twenty Fourteen, you would copy this rule into your custom CSS:
.site-content .entry-header,
.site-content .entry-content,
.site-content .entry-summary,
.site-content .entry-meta,
.page-content {
max-width: 474px;
}
And then change the value for max-width depending upon how much space you want for your content.
If you learn to use a web debugging tool like Firebug or Chrome Developer Tools, you can inspect your own site and see what CSS rules are in effect for a particular element, and write your own overriding CSS.