Resizing slider

How can I customise the slider to the height and width I need? When the theme width is 1100px then the photos are just too big for the rest of the site, but when I use CSS for the sup-slider then I get a grey box around where the rest of the box used to be. Can you please let me know if there’s a possible code I can slip in the custom CSS box or if I need to have a look into the style sheet?