If you go into the galleries/[gallery type]/index.php file, and find a line that looks similar to this:
echo '<img alt="" title="' . $i . '##' . str_replace('"', '\"', $gallery['images'][$i]['title']) . '##' . str_replace('"', '\"', $gallery['images'][$i]['description']) . '" src="' . $gallery['images'][$i]['thumb']['url'] . '" width="62" height="62" />';
You will have to change the width and height there as well. You may also need to adjust some of the other sizes for the gallery to make it look correct.