I'm not totally sure of which Poser render engine you're using. Poser 5 Firefly or Poser 4? One thing I discovered is that the Firefly engine in Poser 5 has many more options, and rather different ways of doing things such as anti-aliasing. The explanation in the manual is a little confusing. In Poser 5, open the Render Options window (Ctrl-Y) and select the Firefly renderer. There are two sliders in the middle of the window, "Pixels samples" and "Post filter size", which are used to set the anti-aliasing. The first tends to have more effect on jagged edges. The second seems to be a controlled blur. As for the original image, there's little point in enlarging an original texture map, unless you have fancy trickery available. But modern characters have multiple maps, insetad of cramming everything into one, and faces will be textured with far more pixels than the resr of the body. If you can make a reasonably good enlargement of the body map, it can help balance the extra pixels on the face.