Poser, as far as I'm aware and I can check with my boss on this...can use textures up to 4096x4096 (previewing textures at that res depends on the strength of your display card - rendering doesn't). 

However, as mentioned earlier, it's rare that this level of detail is needed in day to day use unless you're working for film. Usually 1024 to 2048 will be more than enough depending on the texture.