You can use your browser to inspect elements to find the CSS associated with them. I use and like firebug for firefox.
It looks like it uses a div with id="ProductLongDesc". You could add a class for that anywhere in the CSS to adjust the way it looks.
The large images uses id="lbImage", it sets the image as a background image for the div, so adjusting the size won't be as simple as adjusting the div size, the background image size would have to be adjusted as well.