You can solve this in two ways

No.1 to find the ‘a’ tag with class name ‘ bwg_load_btn_1 bwg_load_btn ‘ in your plugin and add inline style like this

<a class="bwg_load_btn_1 bwg_load_btn" href="https://wordpress.stackexchange.com/questions/285911/javascript:void(0);" style="font-size:16px;">Load More...</a>

2nd in your style.css add font-size for this class with !important,

 .bwg_load_btn_1.bwg_load_btn{
      /* !important will override the style */
      font-size:16px !important;
 }