I found some of the sites that give good information about the different ships...
For comparison of cabin size, our mini-suite on Princess was 280 sq ft and would work for a family. My mom's inside cabin was 156sq ft and would definitely be too small for 4 - the only place for a crib is the small passage between the bed and the desk where the chair is, and the pull-down bunk would overlap some of the queen bed and the passage on one side of the bed.
Cruise411 has rundowns of each ship, including stateroom category descriptions, dining and entertainment descriptions, and photos.
http://www.cruise411.com/c/ShipDetail.asp?lineid=24&vessel=114&i=&d=&d2=&dsc=
http://www.cruise411.com/c/shipdetail.asp?lineid=11&vessel=56
Cruise Critic gives longer descriptions of everything about each ship, and gives ratings based on several categories.
http://www.cruisecritic.com/reviews/index.cfm
Deck plans - sometimes you can find larger cabins within the same category by looking closely at the deck plans. These sites have a complete selection.
http://www.cruisedeckplans.com/DP/Main/home1.php