I'm a day late and a dollar short, but I don't think I made myself clear. It's not so much about the raw size for me, as the efficiency. For instance, my problem with the eye texture I mentioned wasn't that it was 1500 x 1500; it's that it was made of solid blocks of color 6 pixels square. It had obviously been a 250 x 250 map that was resized without even so much as a little smoothing.
I'll admit, one of my guilty texture-making pleasures is 1000 x 1000 eyes. They're fun to make, and that's the size the 400 pixel irises I like to make fit onto. And, every once in a while, I do an extreme closeup that benefits from it.

To me, and also echoing what some others have said, it's about making textures as large as you need, but no larger. If you need 4096, that's fine. It can be scaled down if not needed, but there's no way to regain the detail that's not there. But if you only need 512 and make it 4096 anyway, that's wasteful. And ironically, the texture probably won't look as good as if it were 512! Again, efficiency.

I have a couple of questions kind of related, but I don't know if anyone will have the answers. Does the "max texture size" option also cause P5 to scale smaller textures up? That wouldn't make sense to me, but I think I read somewhere that it did, and since when does Poser make sense?

Also, where to procedurals fit in relation? Unless it's a huge difference in memory, I guess it'll depend on your shader versus the map you could use instead, but is there a rule of thumb? When are you better off going to a node-heavy procedural and when better with a map, if you can get the same effect with each?