Hello Fernanda,

you can specify the following in customizer -> additional css

@media only screen and (max-width: 479px) {
    .flexnav > li > a {
        font-size: calc(1rem + 14vw);
        line-height: calc(1rem + 13.5vw);
    }
}

These are default values, expressed with viewport width units. you can specify the size directly in pixels and add the code to customizer -> additional css.