There are options on export, either "drawing" , "selection" or "page". Drawing will export the entire contents of the file, so if you have some wayward object off to the side of your business card image it will export a box that encompases all content and therefore not be the size you want. If you exprot page it will export the entire page size so if you have one item in the corner of the page you will get your card in a large field of white. Selection will export only what you have currently selected, so if you select your card design, hit export and choose selection it will export as an image (what ever format you choose) only the card design itself.

Clear as mud?