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That is why I thought a good benchmark would be the largest render anyone has ever done. I don't know what that might be... 4000 long side? 6000 long side.

I don't know much about resolution... what is good for an 8x10? 2400x3000? I have always wonder what print size I could get fro my camera that shoots 1200x1600... a 4x5?

Not to be contrary but on an other note, I think I have seen that images loose quality when downsized also. I just do renders at a couple sizes sometime.

But back to the point... say a large render is 4000x6000 would a sensible max texture be twice that, half that, or something else?

I've done renders up to 10,000 x 10,000 pixels... if the renderer doesn't do them, then I render image in pieces and splice it in postwork.

To gage texture size, let's say you're doing a portrait, and the portrait will be 4000vert and maybe 2500 pixel horizontally.  In that case, the face may cover an area up to 3000pix tall, by 2000 pix wide. 
If the texture size used in the portrat, covering the face ara is much smaller then the pixels on the final print, you will be introducing fuzziness. (Much like scaling up an image that doesn't have sufficient resolution.

Some of that can be improved with by adding bump and few procedural tweaks, BUT, that usually happens only with more advanced users.

There isn't a real hard and fast rule that I'm aware of, but the more sharp pixels you have in the textures, the fewer tweaks to get it to look better, and the more flexibility the end user has.

Hi, my namez: "NO, Bad Kitteh, NO!"  Whaz yurs?
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