How do you resize images in the featured category slider of the Blogs MU theme?

I'm using the "featured category slider" option, displaying images in the slider on my home page. I've noticed on one of my installations that the images are smaller (they don't take up the whole area of the slider window) -- and on an older installation I have the images are displayed too large, only a portion of the image shows through the slider window (I think I did some customization to the theme on the older install, but I don't remember what I did, its been too long ago). So I'd like to change the size of the images displayed in the slider window, to be the same size as the slider window. How do I do that? And do I implement it in the custom css file, or somewhere else?