Disney Magic vs Disney Dream
I'm not sure if I got the size comparison right, but deviantart user lupin3ITA created some nice profiles of the Disney Dream and Disney Magic cruise ships. You can see the difference between the first two ships (Magic and Wonder) and the two new ships (Dream and Fantasy).
The most obvious visual difference between the Magic/Wonder and the Dream/Fantasy is the number of decks visible. In the top, white painted portion of the ships the Magic and Wonder have three decks of portholes visible, the newer ships have five. If you can see the aquaduck on top of the ship, you're looking at the Dream (the Fantasy will have it too).
The bow of the Magic has Sorcerer Mickey on it. The stern has Goofy hanging off it and the name of the ship. The bow of the Wonder has Steamboat Willy on it and the stern has Donald hanging from it. The bow of the Dream has Captain Mickey on it and the stern has Sorcerer Mickey hanging from it. The bow of the Fantasy will also have Sorcerer Mickey on it, going by artist renderings. The stern will have Dumbo hanging from it.
Magic and Dream together, don't let the forced perspective fool you, the Dream in back is larger:
The fourth ship, Disney Fantasy just floated out of the dock at Meyer Werft shipyard in Germany! As you can see in the photo there is still construction going on.