The resolution you need depends on how big your render is and how much of the render the skin takes up in the picture.

If you're doing a 3000 x 4000 pixel render of a head shot, 1024 px resolution head textures will not look good. But they'll be fine for a character who is in the distance.

Are you significantly reducing the quality of the character by using smaller resolution base textures? Yes. People who do big renders for print won't be able to use them for portraits. Is that a lot of people? I have no idea. All I know is that the current standard in the marketplace seems to be around 4k x 4k for skin textures, so that's probably what people expect when they buy a character.

On the other hand, since they'll probably be fine for most people who only render for the web, you could maybe market your textures as lower resolution textures for much less memory intensive renders. :)

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PoserPro 2014, PS CS5.5 Ext, Nikon D300. Win 8, i7-4770 @ 3.4 GHz, AMD Radeon 8570, 12 GB RAM.