to change the size of the thumbnails you have to edit the size in the media lib settings and rebuild all thumbnails (wordpress doesn't change the size for images already in the lib, you can use a plugin like:
http://wordpress.org/extend/plugins/...bnail-rebuild/ ), or you can change the width and hight for the img element with css (like: .wGallery img { width: 50px; height: 50px;})
about the centering problem, it would be helpfull if you could post a link...