When it comes to changing up the size of the images there are two ways you can do this, you can edit the css if the actual dimensions of the image on your server are not an issue:
showthread.php?t=11211
If you wan to change the actual size of the images then you will need to edit some of the internal properties for PowerGallery, here is a link to a post that details how to do this:
showpost.php?p=36751&postcount=2