There is no specific option to increase the font size of the category menu sub item. You can try the “Menu drop down second level” from Theme Settings >> Typography >> ADVANCED TYPOGRAPHY but it might disturb many other menus styling.
So you can try the below CSS in Theme Settings >> Custom CSS Global Custom CSS to change the submenu font size of categories menu:
.show-on-hover.header-categories-nav .header-categories-nav-wrap .vertical-navigation .menu-mega-dropdown .sub-sub-menu li a {
font-size:18px;
}
