wordpress搜索的最小符号

It's my searchform.php file. How is it possible to set minimum symbols for search?

<div class="widget-search">
    <form action="<?php echo esc_url( get_home_url() ); ?>/" 
          class="searchform" id="searchform" method="get" role="search">
        <input type="text" value="<?php echo get_search_query(); ?>" 
               name="s" id="s"
               placeholder="<?php _e( 'Search...', 'pixar' ); ?>">
        <button type="submit"> <i class="fa fa-search"></i> </button>
    </form>
</div>