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.
Do you need a balcony? Cruise travelers who spend all their time in the public areas -- sun decks, lounges, restaurants -- or on shore may be perfectly happy with standard-size cabins and no private outdoor space. Those who love to avoid the crowds and lounge quietly on their own verandahs or have private room-service meals outdoors will surely want balconies. Do not forget to take your itinerary into account; on a chilly-weather cruise, you might not be spending too much time outside, so depending on how much space and light you need, a balcony might not be worth the splurge.
Exception: Carnival has several cabins which are classified as inside cabins but actually have a french glass door which allows light into the room (no balcony, but the door can be opened). Carnival also has some cabins that have a window, but because the window has an obstructed view (that means there is a railing or object in the way) it is listed as an inside cabin. Royal Caribbean has some cabins that have a window but look out over an inside promenade area. These are called "promenade staterooms".
When it comes to choosing suite accommodations, it is best to figure out how much space you really need, what amenities are important to you and what you can afford to spend. Suites on most ships are often the first category to sell out, partly because there are fewer of them, and partly because they often offer extremely good value. For this reason, it is important to decide early what kind of suite you would like.