The dimensions of a full size DVD case are 27.5 cm x 18.3 cm. All you need to do is go to "page setup" and select the paper size you use (A4, Legal etc..). Then set the margins so that the above dimensions will center on the page. For example, A4 requires the margins to be set to 1.35 cm left and right and 1.10 cm top and bottom. This will give you the "exact" size of the cover.