adjusting the texture size in poser is not a good idea. poser's dynamic resizing does a much messier job than simply doing it by hand, and it still uses more resources since it is still loading the full size texture map into memory and then just interpolating the detail in the render.

if you really want to save on poser resource usage, then resize the texture in photoshop by hand.

for example (ill use my GND character as an example):

youd open body_nat.jpg (the body tex)  from  ..PoserXRuntimetexturesBlackheartedGND2alpha

the default size of this is 3072x4096
you could adjust the texture size in photoshop to 768x1024 or 1536x2048 and save it as body_natLO.jpg or whatever. in photoshop i would save at around 70% quality compression.

now you can either change the texture from body_nat.jpg to body_natLO.jpg in the material editor by hand, or if you like you can edit the MAT file yourself. to do this simply:

open up windows Notepad
(if you cant find it go to Start > Run > type in notepad.exe and hit enter)
open up the MAT file you want to edit, in this case ..PoserXRuntimelibrariesposeBlackhearted GND2!skin_alpha MAT.pz2

in notepad go to Edit > Replace (or hit CTRL+H)
in the firts field type in body_nat.jpg
in the second field type in body_natLO.jpg (or whatever you named your resized tex)
click 'Replace All'.
save this MAT under a different name, such as !skin_alpha LO.pz2

repeat this for the head tex, etc. you can do this for any texture you like and the results are superior to poser texture resizing. this is especially noticeable around the face.
yes it takes a bit of time, but i would do it if i were creating an animation. by reducing the resolution of the textures of the figure, clothing, hair, etc i would save dozens of hours over the course of a several thousand frame movie. or perhaps if you have a slow computer and regularly render multiple characters, you could do this to the textures of the background characters if they were different from the main one.
for a still render for my gallery? i would not reduce any resolution. the 5-10 min i save on a render due to reducing the textures (either by hand or in photoshop) is not worth it to me.

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but consider this folks -- for preview renders compromising on detail is natural, you bump up the min shading rate to around 3.00 or 4.00, turn down tex res to 500 or so, turn pixel samples to 1, and render a quick small preview to see how the lights and shadows turn out.

but to render a final production render like this? think about the hours it takes to set up a good render, lighting, shadow, pose, etc... then the hours it takes to postwork it or composite it. it even takes time to create thumbs, post it, etc. so the average person is working at least around 3-5 hours on a nice render. why base this all on a 5-minute render that discards 7/8ths of the detail???
my detail examples above (they were not trying for realism, just a quick slapped together render showing the difference in detail between the settings), took 5 min to render the low detail one and 55 to render the high. but this is at EIGHTEEN TIMES antialiasing. if i had a more modest AA setting such as 4-5 it would have rendered in around 15-20 minutes easily.
if you are just slapping renders in your gallery to fill it up and submit something for the sake of submission, sure reduce all the details. if you are animating, you are better served picking up reduced res figures and clothing and manually reducing the textures to 512 or 1024 textures high.
but if you are going to sit there for several hours setting up and then postworking a render, then dont base it on a 5 minute one. go have a coffee, or take the dog out for a walk, and let it render the full detail.

there are many solid suggestions in the first post for rendering such as using the heirarchy editor to get rid of parts of the body under clothing, etc (do not use backface culling however as it will mess up your shadows). but the problem as i see it with the average poser render is that most people do not even change the default poser render or lighting settings for their gallery renders, and they could be much improved by tweaking them for the better, not the worse.