This is a case where context is utterly important. I mean this in a few ways:
- Do you have an existing relationship with this person?
- Are you, at the moment you're thinking about making this comment, already engaged in conversation on a relevant topic?
- Are you phrasing your comment in a way that makes sense, given the conversation?
- Do you have any obvious ulterior motives behind the comment?
If you're talking with a coworker whom you're friends with, and you're discussing her recent travel schedule, it's no big deal (and perhaps even expected) to say something like,
Wow, I'm surprised you can handle all that travel and still be able to bounce back and work effectively today. Last time I traveled for two weeks, I had to take a day off when I returned.
However, even in that context, it may be awkward to say something closer to what you proposed,
You have a great constitution!
Or, perhaps worse, the way you were hinting at phrasing it in your question on the other SE site,
You have a great body!
In other words, when you're already friendly with the person, and talking about their travels, the answer to your question really just boils down to word choice and phrasing.
By contrast, let's look at a different relational and situation context. If you have no real relationship with this person, statements like this will be easy to misinterpret - because your intentions or ulterior motives won't be clear. Even if you're on topic (ie you're talking with her about recent travel, or room temperature), some people may interpret such a broad compliment as an attempt to curry favor, or even initiate a relationship - in short, they may think you're complimenting them simply because you're romantically or physically interested in them. In these cases, it's probably better to just keep your thoughts to yourself. Remaining silent can be especially good advice in the workplace because of the sensitivity to sexual harassment - if this person feels that you're trying to compliment them as a way to pursue a relationship that they don't want, that can easily lead to a slippery slope and given that there's no real upside, it's a bad idea to take that risk.