I think a general comment that you look great nude is fine...but I think if someone starts saying you look fat or your dick is not so big etc. is very rude. Look just because we are nude, does not mean we throw social rules out the window and say whatever you want to someone about how they look. This is common sense. The more you know someone and comfortable then you can be more free but, someone you do not even know there should be some discretion.
Complimentary remarks are always welcome, we all need an ego-boost from time to time, but if you can't say anything nice,then keep your comments to yourself.
For the record, I consider all naked people to be beautiful, they are not trying to hide anything about themselves.
A comment that has stuck with me came in a clothing-optional bar in San Francisco. My body offered an enthusiastic hello to the group, and somebody said to me "A lot of guys' dicks look kind of mean. Yours is friendly." An odd thought, but I've taken it to heart and it's helped with body acceptance.