I'd like to give you some useful response about what I do myself, but I'm a bit puzzled by your question. You used the phrase "render size window", but do you really just mean "render size"? Or are you actually asking about the size of the Preview / Render window within Poser?

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)