Do you mean the size of the overlay that opens when you click on the thumbnail?
You can resize it, but I think you would have trouble centering it. It currently (if I'm talking about the right thing) set to 100% width and 100% height, - but his measurement starts at top left of the screen and ends at the bottom right.
You could change the left and top values, and give it a set (or %) width, but you can not take into account peoples' screen resolutions and sizes. You would need a script like the auto-centering pop-up window script which reads the screen size and calculates top and left based on the calculated centre lnes. Maybe lightbox has some tech how tos?