1,3)  To change the width of the navigation menu, line height, and font size, please, use the following code:

    .mhb .first-row .tabletMobile ul.menu{
            width: 300px!important; /*Tablet width*/
    }
    .mhb .second-row .mobile  ul.menu{
        width: 250px!important; /*Mobile width*/
    }
    .mhb .first-row .tabletMobile ul.menu ul.sub-menu span{
      font-size: 17px!important;
          line-height: 30px!important;
    }
    .mhb .second-row .mobile  ul.menu ul.sub-menu span{
        font-size: 16px!important;
      line-height: 30px!important;
    }

2) The submenu font size is taken from Betheme -> Theme options -> Fonts -> Size & Style: Content.