During the reservation process, unless the category you've selected has gone GTY, you can select a room.
It will either look like this at some point in your reservation process:
Or this:
Clicking "Change" will take you to a window that looks like this:
There (again, as long as your selected category isn't GTY) DCL will offer you a selection of rooms within your selected category. Or you can "choose your own stateroom" and put a specific room number in there. The system will tell you whether or not that room is available.
I would suggest also having cruisecritic.com open in another tab while booking. If you go there click on Disney Cruise Line then whatever ship you're booking and go to deck plans and you'll get a much better look at the floor plan of the ship than on the DCL website.
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