Depends on the situation. If it's just a compliment and you say it as that and is received like that, then it's very okay, if not.. it could backfire.. you have to be cool and not attached or "wimpy" when you say it, too (that should go without saying). But usually it backfires so I'd stick to a normal conversation. You can make compliments like that later on. Or you could, if you're very inclined into complimenting her, going into specifics, like saying: "That blouse / dress fits you so well.. or is it the other way around? You fit it so well and give it such a nice color / shape". Be flirty, enjoy, have fun and make her have fun and enjoy her time with you. Good luck!