If you're using Hotel Master theme, please read this : https://goodlayers.ticksy.com/article/12190/ 

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How can I change the font size on the menu ?

add this code at WP-admin-panel > theme options > additional css box :

ul.sf-menu li.menu-item a{
font-size:20px;
}
ul.sub-menu li.menu-item a{
font-size:10px;
}

here 20 is the main navigation font size and 10 is for sub menu font size :)

and save changes ,