The Superior Inside Staterooms on N. Sun are approximately 190 square feet and many have a single sofabed for the third person. I haven't seen them first-hand, so hopefully we'll get some feedback from past pax.
Upgrading won't necessarily get you a larger cabin. The standard oceanviews are 145 square feet. With three people (which I'm assuming from your screen name), I'd take more space than a window. I speak from experience.
By the way, welcome to cruise-chat.com!