system
Given the following post, respond as if you're commenting on Reddit:
human
[Alien] What's the minimum/maximum size a potential host has to be for a facehugger to impregnate them?

How small or big would a creature have to be for a facehugger to longer be able to latch onto and impregnate it?
gpt
The minimum size is dictated by how much mass the embryo can take from the host without killing it prematurely.  Tissues such as fat, some muscle mass, even parenchymal tissue from certain solid organs like the liver could be a source of biomass for the growing chestburster.  The loss of some of these tissues might prove detrimental in the long term, but of course the host dies from the chestburster's birth before it becomes an issue.  Spike, the dog on Fiorina 161, was probably pushing the lower limits on host size.

There might not be an upper limit, although facehuggers seem to prefer depositing their embryos in the thoracic cavity.  In a very large animal - say, an elephant - the facehugger might not be able to physically reach that cavity through the mouth.  In that case, it might use its acid to create a new fenestra into the thoracic cavity, as one did to penetrate Kane's helmet on LV-426; it take advantage of a different orifice that gives access to the peritoneal cavity rather than the thoracic cavity; or it might look for a different host entirely.

If a facehugger is able to burn its way into a suitable body cavity, there may be no upper limit to the size of the host.  Since xenomorphs take on the characteristics of their hosts, this raises some interesting possibilities.  The prospect of a xenomorph raised in, say, a cloned Tyrannosaurus would have enormous value to our Bio-Weapons Division.