I would vote on any of Royal Caribbean's Voyager class ships. The Explorer, Voyager, Mainer, or Adventurer. Great size Plenty to do. lively pool deck. good shows. I would not suggest an Oasis class for a first cruise. Just got off one, it was nice, shows outstanding but so crowded, always standing in a line and so hard to find a seat. Of course it was sold out.
I agree with the others. At least 7 days. Shorter and its a totally different experience.