Damn this insomnia and damn this flu; I feel cursed to wander the threads of GupShup, haunting them with my comments!
As for the ROH: assuming the performance (you are talking about attending a performance there, and not just some function hosted the the ROH?) takes place in the evening, it would be considered a “semi-formal” (or black-tie) affair. For women, this means wearing a floor/full length evening gown. The gowns themselves can vary in style, shape, and modesty-level.