Hello @netlover,
Thanks for asking. 
You can add following CSS under X > Theme Options > CSS:
.x-searchform-overlay .form-search label {
font-size: 1.5em;
}
if you would like to customize the search text then the same can be done from /wp-content/themes/x/framework/legacy/cranium/headers/functions/navbar.php. However, before making any changes, please make sure to install and setup child theme. After installing child theme, please copy the file from above path and paste under /wp-content/themes/x-child/framework/legacy/cranium/headers/functions/navbar.php.
Thanks.