This CSS should work for the width:
.page-header-content input[type=search] {
width: 300px;
}
Adding CSS: https://docs.generatepress.com/article/adding-css/
This snippet should change the text:
add_filter('get_search_form', 'my_search_form_text');
function my_search_form_text($text) {
$text = str_replace('value="Search"', 'value="CUSTOM SEARCH BUTTON TEXT"', $text); //set as value the text you want
return $text;
}