Hi, Jean-Marc! It's very difficult to give you the exact dimensions of the mini-refrigerators at the All Star Movies resort. I contacted a cast member from the resort this morning, and was told that there may be very slight differences in the refrigerators, and exact dimensions were not available.
In order to help you determine what you may be able to store in the refrigerator, think of a dormitory size unit. The refrigerators I've always received come to just above my knee (I'm about 5'8" tall), and the shelves are not terribly wide. If, for example, you put a full gallon jug of milk in there, you will probably have to lay it on it's side, and you won't have room for a whole lot of other things.
I would say to help you visualize the size of the shelves, the shelves could probably store about a dozen cans of soda, and there's some space on the door of the refrigerator. Anything tall will have to be placed on it's side.
I hope that helps with your planning! Have a terrific vacation!