See advanced settings > Support Mail Template.

For example if you want to increase the mail width you can change the CSS to this:

      .container {
        display: block;
        Margin: 0 auto !important;
        /* makes it centered */
        max-width: 900px;
        padding: 10px;
        width: auto !important;
        width: 900px; }

      /* This should also be a block element, so that it will fill 100% of the .container */
      .content {
        box-sizing: border-box;
        display: block;
        Margin: 0 auto;
        max-width: 880px;
        padding: 10px; }