I usually stick with the adaptive bucket size, but I haven't done a "serious" render* for ages, so hopefully one of the experts will chime in with some advice. In general, upping the bucket size will make the render go faster, up to the point where it runs out of memory... Whether your renderer can take advantage of all those gigabytes depends, I think, on your Poser version and your operating system.

(*In other words, for artistic purposes, rather than checking that something I made works properly.)