The above css is for the entire page overlay, the slightly transparent black overlay. The size of the white overlay is determined by the large image size as defined in the gallery/grid_lightbox/index.php file. Take a look at this thread: showthread.php?t=8844. There are some instructions there on how to change the image sizes and update your gallery to use that size.
If you want to just change the style of the lightbox popup itself, I would use Firebug to inspect the popup, make a note of the css, and update the slimbox.css file accordingly. The black overlay should be ok as is since I would think you still want it to cover the entire page like it currently does.
-justin
