Hi,

Please go to Appearance → Customize → Custom CSS then add the following custom CSS code:

  @media only screen and (max-width: 479px) {
      .penci-menu-hbg,
      .penci-builder-mobile-sidebar-nav {
          width: 220px !important;
          transform: translateX(-220px);
          -webkit-transform: translateX(-220px);
          -moz-transform: translateX(-220px);
      }
      .open-mobile-builder-sidebar-nav .close-mobile-menu-builder,
      .open-mobile-builder-sidebar-nav .penci-builder-mobile-sidebar-nav {
          left: 220px;
      }
  }

Change 220px to the width you want to show.

Regards,
PenciDesign.