How To Decrease Width Of Single Post Container

Hi there,

you're site is using CSS to adjust that width. Look for this in your Customizer > Additional CSS and change the line i have commented:

.single .inside-article, .comments-area {
    padding-top: 28px;
    max-width: 825px; /* Adjust this measurement */
    margin: 0 auto;
    box-shadow: 0 0 11px 0 rgba(0,0,0,.1);
    border-radius: 8px 8px 8px 8px;
}