Textures are stored within Poser in an uncompressed form, so the only way to reduce your memory usage is to reduce the texture's dimensions. How small you can go depends on whether you're doing close-up or full body work, what the lighting is like, how well mapped the object is, and how big your final render will be. You just have to experiment - but remember to keep the high-res version in case you need it later. As Judith said, you can always reduce from a big master, but expanding will lose quality.