Just runned into the same problem, here’s a quick fix:
Add this to the end of the plugins CSS file
plugins/wp-lightbox-2/styles/lightbox.min.css
#outerImageContainer {width: 80% !important;height: auto !important;}
#lightboxImage { height: auto !important; width: 100% !important;}
#lightbox {top: 80px !important;}
#imageDataContainer {width: 80% !important;}
#prevLink {height: 100% !important;}
#nextLink {height: 100% !important;}
Worked for me, hope it helps.
Haven’t tested with small images. It uses only 80% of the screen an the height is auto.
