Take from here: https://athemes.com/forums/topic/change-submenu-font-fontsize/#
Please try to use the following CSS code to change sub-menu font size to 16px, changing font family is more complicated because there is !important keyword in defining it.
You can add CSS code to your site with Simple Custom CSS plugin (just copy CSS code and paste it in Dashboard → Appearance → Custom CSS after installing and activating the plugin). Also you can add CSS code directly to style.css file of your child theme.
#mainnav .sub-menu li a {
font-size: 16px !important;
}
