I think the question was denim shirt, unless the OP mis-typed. Dinner attire is a judgement call. If you're on the Wonder, I'd say a denim shirt would be fine, since that ship is generally more casual. But your definition of long and loose leads me to believe that you would wear the shirt untucked in a casual type way. You won't be turned away from any of the main dining areas as long as you have clothes and shoes on, but I'd say a denim shirt not tucked in would be on the most casual side for the Magic dining rooms. Polos and open collared dress shirts with dockers are typically the way most casual dresser go on the Magic.
