Ah, found a thread that deals with the question of "power of 2" texture sizing.

paraphrased from @4185701 -- if you use a texture with a resolution of 4000px by 2000 px, Poser will use the same memory as for a 4096 x 2048.

I assume, it would be same for a texture of 3000 by 1500. It would still use 4086 x 2048, so you'd be wasting a fair bit of memory. For 4k texture maps, maybe not such a big deal, but why waste any memory?

(More of that thread deals with whether or not square maps are necessary, due to the way GPUs process mipmaps -- at least that's my take on it. But M4/V4 textures are square, so no need to worry about that here, at least.)

Also, I learned during my search that just increasing the size of your texture map by resampling would not increase its resolution. So to keep purists happy, I guess, a 4096px texture size would be preferable to a 4000px one.

Disclaimer: That's what I got out of my quick skim of previous forum posts. If I'm off base somewhere, some please feel free to correct me. :)

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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.