The galleries are designed to work with the image dimensions specified. For instance, if we take the ribbon+fullsize gallery, the top image is proportionally resized to fit in a 782x481 size. If you want to change the image size, just be aware the design might not work as well depending on your images.
In any case, if you open the index.php file for the gallery you are using, try just adding a height="400" attribute to the <img> tag that is created in the "fullsize" div. So, using ribbon+fullsize again, line 58 is where you would add that height attribute.