You have a nice.
Only if you know the person well, I'd think. It might not be out of line to say "Have you lost weight? You look great!" That gives the person some incentive to keep at the program by being rewarded for the effort put in so far. But as StevieLorna pointed out, it's not something we do.
Comments on body shape, even complimentary, can be seen as judgmental, and that is not what we are about at all.
But if somebody has a nice tattoo or particularly fetching jewelry, that would be grounds for a compliment, because these things are meant to be seen and appreciated by others.
I was in the pool recently at our nudist cloths optional club and noticed a couple, where the female had a magnificent pair of boobs. They weren't massive, but just a nice size and perky. What appealed to me was that the lady in question was a mature lady 60 +. They were very natural and certainly not enhanced. I was so impressed with what I could see and nervously asked her if I could ask her a personal question. Yes, was her reply. I said, could you tell me how a lady of a certain age manages to keep her boobs in such a fine shape. The only problem she had with that was referring to her as an older person.
It often better not to say what you might be thinking. I was at a nudist resort enjoying the sun at the pool. A couple came into the pool area and set up their lounge chair in front of me where I could clearly see them. The lady was attractive with a lovely tan with black hair. She had trimmed her pubic hair in the shape of a small heart. I thought it was quite cute. I almost said something, but my better judgement told me otherwise.
Parts is parts, but breaking out certain parts for compliments is driving into the danger zone of objectification and needs to be avoided. I consider this good advice both inside & outside a nudist resort or beach. My, what a lovely nutsack you're sporting, Santa! It's bad etiquette in almost every circumstance unless you know the person. And you'd better be really good friends if it's body parts you're complimenting.
In 20+ years of social nudism, I've only complimented one person on "parts." BUT she happened to be a longtime nudist friend, and this was after we'd known each other - and hung out naked - for quite a while. She was a fifty-year-old woman with "gravity defying breasts"; and I felt we were in a place where a compliment would be well-received and appreciated. And it was.
Beyond a situation such as that one; I can't think of another where such a pointed compliment would be appropriate. Weight-loss, all-over tan, nice tattoo...ok; after you've gotten to know the person. But I feel you'd have to know the person VERY well before being more specific.