What do you suggest decreasing the maximum texture resolution to?

Another thing you can try, if you don't mind some extra work, is to make the texture files themselves smaller. I have the high-res texture set from DAZ for Michael 3, which is nice but a waste of resources if he's very far from the camera. I took all of the high-res textures, made copies of them, made the copies 25% the size of the originals, saved them with the letters "Hi" in the file name changed to "Lo", and made a copy of the Mat pose file with the new names in it, so that I use less memory with a lot of Mikes or with Mike standing far away.

Generally speaking, I think you'll get pretty good results if your texture files are no larger than about 125% of your rendered image size, even on a close up. So, if you're rendered image is going to be 1024 by 768, you don't really need a 4000x4000 texture map, as 1200x1200 one will probably work just as well.

Is there no patch for this problem?

Poser 5 is probably about as patched up as it's gonna get. My guess is some of these issues will probably be addressed in Poser 6, if and when such a critter gets released. The owners of Poser have had their ups and downs with the product, to be sure.