Through massive experimentation and a LOT of blind luck, I finally found it. For anyone who needs to know the answer to this puzzle, I hope this helps.

includes/templates/YOUR_TEMPLATE/css/index_home.css

Lines 390-395

.nivoSlider {
position:relative;
width:40%;
height:auto;
left:160px;
overflow: hidden;

Width:40 shrinks the size of the graphic; left:160px centers it (at least on my template--it might be slightly different on other ones).