Hello
Presuming you are referring to the logo text, try adding the following to the Custom CSS field of your templates parameters (edit to taste)...
@media only screen and (max-width: 959px) {
h1.logo-text a {
font-size: 40px;
}}
@media only screen and (max-width: 767px) {
h1.logo-text a {
font-size: 30px;
}}
@media only screen and (max-width: 440px) {
h1.logo-text a {
font-size: 20px;
left: 0;
}}
CiarĂ¡n