Agree with LaurieA on this (she beat me to the post, but I'm posting anyway)

I think the most appropriate answer must be: the largest that you can fit into your screen size, leaving space for the tools & palettes around it.

 

I have a 1680x1050 pixel display for my main screen, and 1280x1024 for my second screen. I use the second screen for the library palette. My document window is a landscape layout 1125x750 pixels.

 

If I'm working on an image that is not that particular size or aspect ratio (like the test render in the image above) I just change the render dimensions, but I generally keep the document window the same size. If you wanted to, you could easily change the document window size to suit your image, and you can save your commonly used layouts using the UI memory dots.

 

Not sure if this helps you at all!

(edit) regarding the render itself, I totally agree with what ockham says. The best way to get more detail is to render more pixels!

 

"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)