Poser 7 and up use a different texture cache mechanism during the render where texture size does not matter too much. (It uses a predetermined amount of memory). That is why the max texture size box was removed from the firefly settings.

Max Texture size in the Preview settings influences how much memory Poser uses for textures in the preview mode. Reducing that gives you more memory. Higher gives you better preview quality but reduces the amount of memory available for the rest of Poser

So for Poser 7 and up you do not need to lower the resolution of texture maps