Cabin 6A's are the catagory of cabins you are in, as far as the cabin # it depends if you have assigned cabins or guarantee cabins. The numbers are usually ex: R1243 or M2247.
The catagory 6A's on this ship are on Riverea deck, 6b's on Main decks and 6E's are on Deck 10 Spa deck. I posted earlier that Carnival does not have adjoining cabins, that is on the older ships, the Destiny does have adjoining cabins so its according to your cabin number if you do. If you have a balcony they are usually catagory 8, 9, 11 or 12.
As far as the size not sure how to convert to meters but the size is around 185 sq. feet.
Your will have assigned dining based on whether you chose early or late seating. You will have an assigned table for dinner each night, ( I like this as you can eat with the same people each night) for breakfast and lunch it is open seating. Of course there are other options such as the 24 hour pizzaria, or the buffets for breakfast or lunch and also for dinner if you choose not to dine in the dining room. There is also an alternative dining option for a small fee if you prefer that. You will get your table assignment when you check in at the port.