u can keep the settings to customize and alcohol will detect itself what kind of image to be created, but i will suggest u to select .iso bcos the size of iso image is same that of total size of CD contents but if create other images like mds , cue, ccd etc their size will be bigger . for eg mds image of 670 MB cd will be of size around 730 MB but if u create iso image its size would be same Ii.e. 670 MB.