you can find this code in file script.js

function BannerSlider() {
        if($('#banner-slider').length) {
            var offset_h = $('#header').innerHeight();
            $('#banner-slider').owlCarousel({
                autoPlay: 5000,
                navigation: true,
                singleItem: true,
                pagination:false,
                transitionStyle:'fade',
                navigationText: ['<i class="hillter-icon-left-arrow"></i>','<i class="hillter-icon-right-arrow"></i>'],
                beforeInit: function () {
                    var height = $('#banner-slider').data().height,
                        window_h = $(window).height(),
                        window_w = $(window).width();


                    $('.slider-item').each(function(index, el) {
                        var url = $(this).data().image;

                        $(this).css('background-image', 'url('+url+')');


                        if( height !='' && height != undefined ) {


                            if(window_w > 767 ) {
                                $(this).css('height', height);
                            } else if(window_w <= 767 ) {
                                $(this).css('height', 500);
                            } else if(window_w <= 480 ) {
                                $(this).css('height', 400);
                            }

                        }else {
                            $(this).css('height', window_h - offset_h);
                        }


                    });


                },
                beforeUpdate: function() {
                    var height = $('#banner-slider').data().height,
                        window_w = $(window).width();


                    if(!(height !='' && height != undefined)) {
                        $('.slider-item').each(function(index, el) {
                            var window_h = $(window).height()
                            $(this).css('height', window_h - offset_h +'px');
                        });
                    } else {
                        $('.slider-item').each(function(index, el) {
                            if(window_w > 767 ) {
                                $(this).css('height', height);
                            } else if(window_w <= 767 ) {
                                $(this).css('height', 500);
                            } else if(window_w <= 480 ) {
                                $(this).css('height', 400);
                            }
                        });
                    }
                }
            });
        }
    }

 change it with the following code: 

/*Banner Slide*/
    BannerSlider();
    function BannerSlider() {
        $("#banner-slider").owlCarousel({
            autoPlay : 3000,
            stopOnHover : true,
            navigation:true,
            paginationSpeed : 1000,
            goToFirstSpeed : 2000,
            singleItem : true,
            autoHeight : true,
            transitionStyle:"fade"
          });


        $( ".slider-item" ).each(function() {
          var attr = $(this).attr('data-image');


          if (typeof attr !== typeof undefined && attr !== false) {
              $(this).css('background-image', 'url('+attr+')');
          }
        });


    }

and use css with class 

.banner-slider .slider-item

 change the layout size especially the hight of banner slideshow on dekstop Or mobile. Example:

.banner-slider .slider-item {
    height: 500px;
}

 Thanks