I am wondering which tag to use for questions specifically relating to the physical capabilities of humans in space.
human could be applied to the activities of humans in space, but we already have manned-flight so I don't think it's a useful tag at the moment, unless we are talking about human colonization of space (but all the questions I can think of would fit under human-physiology).
There is a term and scientific discipline called space-medicine. In my eyes, this makes sense for most humans-in-space-related questions. health does not fit completely. I could imagine events in space (observed by exploration activities) that have an influence on the health of people here on Earth, too. So for phenomena on human bodies in space, it is too wide. human-physiology is also nice, but I think that this is useful for rather specific questions.