I have no advice on the topic. Particularly not when it comes to Mental Ray (I've not looked into any of it yet). I have no experience with high poly or insanely large textures. I think 1024x1024 is the largest I've ever worked with, and I think my machine would choke with 4096x4096's hehe. I hope some of the lads with more experience will chip in.

It is my understanding that the final output resolution of your work (film) will be the deciding factor, and that you have to compress to match the aspect ratio (2.35 to 1 or whatever you choose).
Here's a snippet from Maya Killer Tips 6:
"A texture map should ideally match the screen space of what it occupies in the render, but animation and perspective alter that simplicity. A good rule of thumb is to find the nearest proximity of the texture in relation to screen resolution and then scale down the resolution 75%. Texture aliasing should be avoided like mad cows, so shooting a bit smaller than screen resolution will ensure some stability in the texture, which will get visually crisper in animation anyway. Also, smaller textures equals less RAM and shorter render times"

Texture artist Leigh van der Byl has some clear thoughts on the topic that will help combine all this info
https://www.leighvanderbyl.com/pdf/size_does_count.pdf