in the galleries folder, open the subfolder for the gallery you are using then open the index.php file.
locate the line that initializes the gallery:
//process gallery
$gallery = get_gallery(get_mediarss_url('gallery=' . $pg_id . '&thumb=62c&full=995c'));
and the full argument to use an image width that will fit your design:
$gallery = get_gallery(get_mediarss_url('gallery=' . $pg_id . '&thumb=62c&full=782p'));