Do you have to have a whirlpool bathtub or a walk-in closet? Will you be entertaining and thus in need of a dining table that can seat six or eight? Do you want benefits like priority dinner reservations and being first in line to get on or off the ship? Do you want to be pampered with extra-plush linens and bathrobes, fancy bath products and in-suite coffee and booze? You can find those amenities and more in most of the upper-level suites.
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.
In order to set the pricing many cruise lines will divide each cabin type into sub categories. For example a balcony cabin may be divided into sub categories such as B1, B2, B3, B4. In most cases the cabin size and features in a sub category is the exactly the same. The only difference is usually the location of the cabin on the ship. Please look for the notes on the category descriptions to determine if there is a difference in size or features between the subcategories.
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.