I think the print volume of an Ender-3 is generally a good starting size, and you can look at getting bigger from there. I have a CR-10s which has a pretty decent print volume (especially vertically). I've never printed armour on it, but I hardly need to push the limits with fitting stuff on there. What you may want to ask yourself is, are you prepared to dish out a bit more on a bigger printer to save time not having to slice and glue as many parts?
For adults, a drastic difference in body sizes between users wouldn't really equate to a drastic change in build plate requirements.