I am not sure I understand your question; which dimensions and aspect ratio... Shozam generates all images to fit within predefined dimensions. You may be able to somewhat alter the layout by operating on the theme's CSS file. This editing though will need to be outside of Shozam; the simplified theme editor within Shozam's Step 4 does not provide access to those dimension.
Internally we are currently experimenting with a fluid layout (that expands or contracts depending on the size of the viewer's browser window) - if workable we will release as a downloadable theme. However, images will retain their predetermined sizes for now; those sizes are hard-wired into Shozam.