Speaking from general cruise experience, there will be bigger tables, and there will also be some tables for two. Tables for two are usually kinda hard to get, but I suppose that depends on the particular ship. I'm usually fairly anti-social, but I think it's great fun to sit at a larger table, maybe for 6 or 8, and get to know new people over the course of the week. You come in to dinner at the end of the day and share your experiences and they will share theirs. It's all very interesting. But you can always talk to the maitre d' as soon as you board and ask about a table for two.
Regular dining room attire is fairly casual on the Carnival ships. Jeans are allowed. Carnival just wants to keep it a clean-looking environment. On formal night, they suggest a sport coat or dress slacks and a dress shirt. Wear a tux if you like. I've seen peoplel do that. Some of the women like to really dress it up on formal night. Me, I'm a jeans and t-shirts kinda gal, so I clean it up a bit for the dining room (polo and jeans). On formal night, I tend to wear something I might wear to church on Sunday. Or out to a night at symphony. Not a formal dress, but glammed up just a bit.