I'm not in front of Poser at the moment, but  both options are on the bottom of the Render menu.

There's a checkbox textures and another I think is called "Reuse Shadow Maps". 

There are entries next to them to reload textures (clear the cache) or clear the saved shadow maps. So basically once either gets created Poser won't load or build them again.

I start out with my Max Texture Size at 1K. I do many renders without shadows. 

Then I enable shadows and if I've moved anything I clear the shadow map cache. Then I do a check render
Then I change my max texture size to 4K, click reload texture (because I want it to start using 4K not the old 1K), start the render, then go get lunch.

4K textures seem to be the limit of Poser on my machine, and my PC gets a little ill after using them. 3K works fine almost all the time, but there is some loss of detail on hair. If you're not doing a hair closeup, 3K is more than enough.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)