For your requirements, ignore the above code and, just do the following since you have already set the logo size right through the theme options setting provided –
@media only screen and (max-width: 479px) {
#header-logo, #site-title a { width: 200px !important; height: 100px; background-size: 200px !important;} /* Reduce size for smartphones only */
}
Made this change on your site, checked on my iPhone and looks fine.