Maybe say thin, slim, or slender. Too many fatties have usurped curvy, and if I saw curvy as a descriptor I wouldn't contact them. I'd also not use the word healthy, because it's dodging the question, and like with any euphemism, if you have to dodge that question, it still tells me what I need to know. Average or athletic can be hit or miss, since a lot of people with nice bodies can still be self-conscious, but fatties will rarely be honest enough to say fat.