Where is she cruising to? Do you want an ocean liner-style ship or a regular cruise ship? Cunard line, which has the Queen Mary 2, Queen Victoria and the recently retired Queen Elizabeth 2, is the last of the class-based lines. There are certain restaurants and lounges open only to those cruising in higher cabin categories. This is mostly just a nod to history these days, with 99% of the ship open to all....but something that could give your story a little extra flair.
On any ship, of any line, there are a wide range of cabin categories. The older the line, the more obvious differences between the cheap cabins and the more expensive ones. On a newer mass market line like Carnival, all cabins of a specific category (inside, outside, balcony, suite) are basically the same, with those on higher decks costing more. A handful of huge penthouse suites are usually available, perhaps with private butlers, and are breathtakingly expensive.
If your MC isn't in a penthouse suite, expect space to be at a premium. Depending on the age of the ship, average size w/ensuite is anywhere from 125 to 185 sf. There are generally two twin beds that can be pushed together (the cabin steward will do this). Some cabins have upper bunks that fold into the wall.
However, every inch of the cabin is carefully designed to maximize space. There's plenty of storage and plenty of room to move around. On my most recent cruise with my father, we were able to get his ECV into a regular inside cabin and still had room to comfortably get around.
You'll have a cabin steward, whose duties are similar to housekeeping at a nice hotel several years ago. He or she will greet you within a few minutes of arrival and ask about your preferred bed arrangements and dining time. They're sometimes known as magic elves, since they always manage to slip in and tidy up as soon as you leave the cabin. When you return from dinner, your bed will be turned down with chocolates and a copy of the next day's ship's newsletter. There will be one or two cute animals made of towels nearby. The steward is also happy to handle simple requests, such as keeping your ice bucket filled.
Cruise ship toilets are something to behold. The flushing power is, let's just say, powerful! A lot of people jump the first time they flush, and there's usually a joke made about it at the Welcome Aboard show.
If you have any specific questions, feel free to PM. I'm an avid cruiser and always happy to discuss my experiences.