You can change the main font-family of the theme by clicking on Appearance -> Theme Options -> Advanced Settings.

To customize the font-size, please put your Custom CSS to Appearance -> Theme Options -> Advanced Settings -> Custom CSS.

Some suggestions for you:

Menu Font-size:

#wilcity-menu .menu-item{
font-size: 14px;
}

Quick Search Form Font-size:

.header_searchText__10VVZ{
font-size: 14px;
}

Listing Font-size:

.listing-detail_body__287ZB{
    font-size: 18px;
}

Footer Font-size:

.footer_module__1uDav .menu-item, .footer_module__1uDav.widget-subsc__text{
font-size: 14px;
}

Copyright Font-size

 

.footer_textWrap__Xc_Ht.wil-text-center .footer_text__1FkcM{
font-size: 14px;
}

Heading Font-size

 

h1{
    font-size: 14px;
}
h2{
    font-size: 14px;
}
h3{
    font-size: 14px;
}
h4{
    font-size: 14px;
}
h5{
    font-size: 14px;
}
h6{
    font-size: 14px;
}

You can also use Google Console to generate Custom Code yourself: