Larger Lightbox(PrettyPhoto) in mobile/tablet

Try to put this code in additional css in admin panel or at the end of style.css

@media only screen and (min-width: 480px) and (max-width: 767px) {
	 .pp_pic_holder{ left: 50% !important; width: 400px !important;  margin-left: -200px !important; }
	 div.pp_default .pp_content_container .pp_right{ padding-right: 21px !important; }
	 .pp_content, #pp_full_res img{ width: 100% !important; height: 100% !important; }
	 div.pp_default .pp_content_container .pp_details {margin-top: 20px !important; }
}

@media only screen and (max-width: 479px) {
	 .pp_pic_holder{ left: 50% !important; width: 300px !important;  margin-left: -150px !important; }
	 div.pp_default .pp_content_container .pp_right{ padding-right: 21px !important; }
	 .pp_content, #pp_full_res img{ width: 100% !important; height: 100% !important; }
	 div.pp_default .pp_content_container .pp_details {margin-top: 20px !important; }
}

Hope this help :)