Making the site fixed-width

Open the styles.css file located in the skins/[CUSTOMER_ACTIVE_SKIN]/customer directory of your CS-Cart installation, where [CUSTOMER_ACTIVE_SKIN] is an active skin of your storefront.

Find the following part of the code there:

body {
    background-color: #ffffff;
    min-width: 910px;
}

and replace it with this one:

body {
    background-color: #ffffff;
    width: 1024px;
    margin-left: auto;
    margin-right: auto;
    position: relative;
}

where replace 1024 with the desired value.