Following past research examining online advertising design and effectiveness, this research studies the impact of variations in the design of online banner advertisements on advertising involvement and effectiveness. Advertisement involvement and effectiveness are measured as response to changes in message design and are determined by the number of clicks on the banner ad (involvement) as well as the click-through rate, or CTR (effectiveness). The latter is the ratio of ad clicks to its total impressions. Related to the message design, the type (static or dynamic), size (pixel ratios), and the format of a banner advertisement are studied employing behavioral response data from a single apparel retailer. Results suggest that the type of banner advertisement significantly influences advertising involvement and effectiveness. Results also suggest that banner ad size in terms of pixel ratios significantly increases advertising involvement through total number of clicks but does not affect effectiveness through CTR. Our findings also identify and empirically validate the important role of the Golden Ratio in banner ad message design and its effectiveness.