On many ships, basic inside and outside cabins are usually the same size, the difference being that one has a porthole or picture window to let in natural light. Balcony cabins can also be the same size as standard insides and outsides, with the addition of the outdoor space on the verandah; sometimes the interior space is larger. A basic cabin, regardless of category, is referred to as a "standard" unless there is something about it that makes it different (such as physical layout, being handicapped accessible or a designated family cabin). With minisuites on up, you get bigger and bigger indoor and outdoor spaces.
Very few ships actually have cabins dedicated to solo travelers. These will have sleeping space for one and can be quite small. The studio cabins on select Norwegian ships are the most famous example of this: The 100-square-foot staterooms each contain a full-size bed, nifty lighting effects and a large round window that looks out into the corridor. If you are a solo traveler, you will want to price out the cost of a solo cabin (usually somewhat higher than the double-occupancy rate of a similarly sized stateroom) compared to the cost of paying the single supplement (an extra fee tacked on if there are not two people in a cabin; the price can come out to as much as double the regular rate) for a standard cabin. And book early, as solo cabins sell out quickly.
Inside cabins with no views at all are typically the smallest, cheapest cabins onboard. They are great options for budget-minded travelers who do not intend to spend a lot of time in their stateroom, or who want to sleep all day in absolute pitch dark. They are less ideal for cruisers prone to seasickness, those who need natural light and groups who require a lot of in-cabin space. Not everyone will be happy in an inside cabin; it is worth upgrading if the lack of light will put a damper on your vacation.
If scenery is important to you, take a good look at your cruise itinerary before selecting your cabin, specifically if you are choosing an outside or balcony. On a roundtrip Caribbean cruise or a transatlantic crossing, for example, the side of the ship you are on does not really matter. If, on the other hand, you are doing a one-way sailing (such as a southbound Alaska cruise or a trip from Barcelona to Rome), you might want to consider choosing a cabin on the side of the ship that faces the land. Sometimes the views can be breathtaking, and you would not get those views from the cabins that face out to the open sea.
carnival cruise room layout
stateroom on a cruise ship
carnival cruise rooms for 5