If you need to modify the size of your gallery that is on the page you will need to adjust the width of it via css. For example, with the full size image on top you can adjust the width in this class:
For the size of the thumbnail wrapper you can adjust it with this class:
.pg-rf .ribbon .thumbs-wrapper
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:
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:
To adjust the time for the slideshow you can reference this How To here:
This document and more are located on the PowerGallery support page here:
As far as integrating PowerGallery with your existing db and user experience this would be a custom implementation. We do not have any guides or documentation on this. The gallery is designed to just display images that you add and manage through the admin area.