To make the lightbox responsive open the plugin folder and edit the file form-lightbox.php and replace the code from line number 113 to line number 120 with the below code
$hidden_output .= 'jQuery(".fl_box-' . $id . '").colorbox({
inline : true,
maxWidth: "90%",
onComplete : function() { jQuery(this).colorbox.resize(); },
href :"#form-lightbox-' . $id . '",
' . $flb->get_lb_opt('colorbox') . ',
onClosed : function(){ jQuery("#form-lightbox-' . $id . ' iframe").attr("src", iFrame_' . $id . '); }
}';
$hidden_output .= ');
// resize colorbox on screen rotate in mobile devices and set to cover 90% of screen
jQuery(window).resize(function() {
jQuery.colorbox.resize({width:"90%"});
});';
Hope this helps someones time 🙂