Illustrating oversized will yield lots of detail, especially in the faces and hands, but will add a step (and additional costs) to the production process. Oversized illustrations are photographed by a professional photographer, then the transparencies or negatives are scanned. Other art that needs photography is collage illustrations such as cut paper work. One illustrator I read about would paint a 30x40" oil on canvas for book covers measuring 5x8". If you decide to work oversized, allow for the darkening factor. When art is reduced in size it appears darker, so keep your paint colors light.