I think I got it. This gives me proportional sizing that covers the entire area and centers the photo. Looks good for the horizontal photos but looks like crap for square logos. Definitely going to have to curate the photos for the best look. (Featured listings will have separate logo areas. So, hopefully, the featured photo will be a an actual photo.)
Just out of curiosity, does using the ID for these stop these styles from being used on the slider elsewhere? (Sorry for the CSS newb question.)
#listing_slider_view-9 .geodir-listing-slider-widget {height: 500px;}
#listing_slider_view-9 .geodir_flexslider .geodir-slides img {width: 100%; max-height: 100% !important; object-fit: cover;}
#listing_slider_view-9 .geodir-slider-title {font-size: 24px; background: #f47a20;}
#listing_slider_view-9 #geodir_widget_carousel.geodir_flexslider.flexslider-ltr {display: none;}