Sonny is on the money here... Carnivals inside, oceanview and balcony cabins are all the same size (185 sq ft) not counting the balcony. There are some in each category that are bigger/smaller because of their location (like corner cabins or cabins near the front of the ship where it gets narrow) but for the most part they are the same size. If you want more space you have to move up to a suite, but that is considerably more costly than an inside.
As long as you booked an inside cabin that sleeps four you will be fine. And like Sonny said, you don't spend much time in your cabin anyway. Sleeping, bathing and changing clothes is about it. Bring a plug in night light though. The inside cabins are pitch black once you turn out the lights, we bring a night light so that there is some light in the room in case you need to get up in the middle of the night.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007