Keep in mind that pools, public areas, and facilities are scaled to the ship's size. Your destination may be a factor in selecting ship size. For example, some of the newer ships are too large to fit through the Panama Canal. Less popular destinations are served by smaller ships. First decide if you want an inside or an outside cabin.
Outside cabins offer a window.
Inside cabins have a mirror, picture, or curtains in place of a window. Less expensive, inside cabins offer the same facilities and service but make some people feel claustrophobic. On newer ships, inside and outside cabins tend to be similarly sized. On converted ocean liners, however, the outside cabins are almost always larger. This varies with individual preferences.