Again, there is much confusion on this. Neither the passporter or the actual docs have good solid answers on this. However, I wrote up a good answer based on info form here, I'll cut and paste it.
Ok, you have a few rules you have to meet to bring it aboard.
First, NO ice! Ice can mess up the scanner and they will make you dump it all out before letting you in. Don't worry, ice is available on Deck 9 at the drink station and your room steward will bring as much as you want, up to three times a day!
Second, NO hardsider coolers(think the 'igloo' cooler), that's what they are really not allowing.
Third, The size of your carry on must fit through the scanner to get into the terminal and on the ship after excursions. Disney uses an airport type scanner. The average size for one to fit is 22 inches long by 14 inches wide by 9 inches thick. It can't be much bigger than that.