I think it's quite difficult to translate"大气" into a single word...if u say a girl is "大气的", u don't wanna describe her as cute, sweet, blabla, she has a stronger air. It's something about people's character, behavior and the impression they leave us. For instance, if u say taylor swift is sweet, then modonna is much much more "大气”。 Anyway it's a positive word, if someone describe me as "大气", i'll be very happy:)