Lightbox size

Hi

I have installed the plugin. It looks great except I am having a newbie issue with size. I have a loop where the featured image is showing. In order to have the image on the loop fit into its container I have decreased the size of the image to something like 100-150px width and 150-200px height. The problem I am having it that the image in the light box is the same size when the loop image is clicked and I need it to be much bigger so people can read text in the image.

I know this sounds a bit dumb but I can’t get round it. Any help is greatly welcomed

http://wordpress.org/extend/plugins/lightbox-plus/