Category 5,6,7,8,9 &10 are all the same sized rooms inside (excluding verandah space).
The differences -
Cat 10 are inside rooms,
Cat 9 are oceanview on decks 1 & 2,
Cat 8 are oceanview on decks 5,6,&7,
Cat 7 are Verandah rooms with partial view obstruction towards the back of the ship on decks 5,6,&7
Cat 6 are Verandah rooms on decks 5 & 6
Cat 5 are Verandah rooms on deck 7
Category 11 & 12 are smaller than 5 through 10 and I think would be crowded for the 4 of you.
Inside-ocean view-verandah is a matter of preference and budget. We like the verandah just to sit out and watch the ocean. It certainly isn't necessary. The porthole in all the ocean view rooms (except deck 1) is quite large and gives an excellent view. Some of my kids love the inside rooms and don't miss not having a view. So really it is a decision based on personal preferences. Same with location - some people love deck 2, others like to be on higher decks. Theoretically, ship motion should be less on deck 2.
A few specific recommendations
If you want a verandah, choose Cat 6 over Cat 5. The extra money for Cat 5 just to be one deck higher is not worth it in my opinion
If you decide on Cat 9 - choose deck 2 over deck 1. Deck one rooms have two smaller portholes vs. the one large one on deck 2
If you are flexible on dates, try the off season dates from September through Feb (excluding Holidays) Rates are significantly lower. We often go right after Thanksgiving and have good rates, good weather and a ship decorated for Christmas!
There are several sites to see the room configurations. I know you can go to DreamsUnlimited site shown at the the top of this page and look for "Staterooms" and "Deck Plans" on the right side of their page.